fix(#411): QA-002 — invert verifySession error classification + health check escalation

verifySession now allowlists known auth errors (return null) and re-throws
everything else as infrastructure errors. OIDC health check escalates to
error level after 3 consecutive failures.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/auth/auth.service.spec.ts

@@ -161,6 +161,8 @@ describe("AuthService", () => {
       (service as any).lastHealthCheck = 0;
       // eslint-disable-next-line @typescript-eslint/no-explicit-any
       (service as any).lastHealthResult = false;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).consecutiveHealthFailures = 0;
     });
 
     it("should return true when discovery URL returns 200", async () => {
@@ -252,6 +254,90 @@ describe("AuthService", () => {
       expect(result2).toBe(false);
       expect(mockFetch).toHaveBeenCalledTimes(1);
     });
+
+    it("should escalate to error level after 3 consecutive failures", async () => {
+      const mockFetch = vi.fn().mockRejectedValue(new Error("ECONNREFUSED"));
+      vi.stubGlobal("fetch", mockFetch);
+
+      const loggerWarn = vi.spyOn(service["logger"], "warn");
+      const loggerError = vi.spyOn(service["logger"], "error");
+
+      // Failures 1 and 2 should log at warn level
+      await service.isOidcProviderReachable();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0; // Reset cache
+      await service.isOidcProviderReachable();
+
+      expect(loggerWarn).toHaveBeenCalledTimes(2);
+      expect(loggerError).not.toHaveBeenCalled();
+
+      // Failure 3 should escalate to error level
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0;
+      await service.isOidcProviderReachable();
+
+      expect(loggerError).toHaveBeenCalledTimes(1);
+      expect(loggerError).toHaveBeenCalledWith(
+        expect.stringContaining("OIDC provider unreachable"),
+      );
+    });
+
+    it("should escalate to error level after 3 consecutive non-OK responses", async () => {
+      const mockFetch = vi.fn().mockResolvedValue({ ok: false, status: 503 });
+      vi.stubGlobal("fetch", mockFetch);
+
+      const loggerWarn = vi.spyOn(service["logger"], "warn");
+      const loggerError = vi.spyOn(service["logger"], "error");
+
+      // Failures 1 and 2 at warn level
+      await service.isOidcProviderReachable();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0;
+      await service.isOidcProviderReachable();
+
+      expect(loggerWarn).toHaveBeenCalledTimes(2);
+      expect(loggerError).not.toHaveBeenCalled();
+
+      // Failure 3 at error level
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0;
+      await service.isOidcProviderReachable();
+
+      expect(loggerError).toHaveBeenCalledTimes(1);
+      expect(loggerError).toHaveBeenCalledWith(
+        expect.stringContaining("OIDC provider returned non-OK status"),
+      );
+    });
+
+    it("should reset failure counter and log recovery on success after failures", async () => {
+      const mockFetch = vi
+        .fn()
+        .mockRejectedValueOnce(new Error("ECONNREFUSED"))
+        .mockRejectedValueOnce(new Error("ECONNREFUSED"))
+        .mockResolvedValueOnce({ ok: true, status: 200 });
+      vi.stubGlobal("fetch", mockFetch);
+
+      const loggerLog = vi.spyOn(service["logger"], "log");
+
+      // Two failures
+      await service.isOidcProviderReachable();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0;
+      await service.isOidcProviderReachable();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (service as any).lastHealthCheck = 0;
+
+      // Recovery
+      const result = await service.isOidcProviderReachable();
+
+      expect(result).toBe(true);
+      expect(loggerLog).toHaveBeenCalledWith(
+        expect.stringContaining("OIDC provider recovered after 2 consecutive failure(s)"),
+      );
+      // Verify counter reset
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      expect((service as any).consecutiveHealthFailures).toBe(0);
+    });
   });
 
   describe("getAuthConfig", () => {
@@ -349,14 +435,72 @@ describe("AuthService", () => {
       expect(result).toBeNull();
     });
 
-    it("should return null and log warning on auth verification failure", async () => {
+    it("should return null for 'invalid token' auth error", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue(new Error("Invalid token provided"));
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("bad-token");
+
+      expect(result).toBeNull();
+    });
+
+    it("should return null for 'expired' auth error", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue(new Error("Token expired"));
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("expired-token");
+
+      expect(result).toBeNull();
+    });
+
+    it("should return null for 'session not found' auth error", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue(new Error("Session not found"));
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("missing-session");
+
+      expect(result).toBeNull();
+    });
+
+    it("should return null for 'unauthorized' auth error", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue(new Error("Unauthorized"));
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("unauth-token");
+
+      expect(result).toBeNull();
+    });
+
+    it("should return null for 'invalid session' auth error", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue(new Error("Invalid session"));
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("invalid-session");
+
+      expect(result).toBeNull();
+    });
+
+    it("should return null when a non-Error value is thrown", async () => {
+      const auth = service.getAuth();
+      const mockGetSession = vi.fn().mockRejectedValue("string-error");
+      auth.api = { getSession: mockGetSession } as any;
+
+      const result = await service.verifySession("any-token");
+
+      expect(result).toBeNull();
+    });
+
+    it("should re-throw unexpected errors that are not known auth errors", async () => {
       const auth = service.getAuth();
       const mockGetSession = vi.fn().mockRejectedValue(new Error("Verification failed"));
       auth.api = { getSession: mockGetSession } as any;
 
-      const result = await service.verifySession("error-token");
-
-      expect(result).toBeNull();
+      await expect(service.verifySession("error-token")).rejects.toThrow("Verification failed");
     });
 
     it("should re-throw Prisma infrastructure errors", async () => {