fix(api): MS22 Phase 1 post-coding audit (#625)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/api/src/chat-proxy/chat-proxy.controller.ts

@@ -1,4 +1,14 @@
-import { Body, Controller, Post, Req, Res, UnauthorizedException, UseGuards } from "@nestjs/common";
+import {
+  Body,
+  Controller,
+  HttpException,
+  Logger,
+  Post,
+  Req,
+  Res,
+  UnauthorizedException,
+  UseGuards,
+} from "@nestjs/common";
 import type { Response } from "express";
 import { AuthGuard } from "../auth/guards/auth.guard";
 import type { MaybeAuthenticatedRequest } from "../auth/types/better-auth-request.interface";
@@ -8,6 +18,8 @@ import { ChatProxyService } from "./chat-proxy.service";
 @Controller("chat")
 @UseGuards(AuthGuard)
 export class ChatProxyController {
+  private readonly logger = new Logger(ChatProxyController.name);
+
   constructor(private readonly chatProxyService: ChatProxyService) {}
 
   // POST /api/chat/stream
@@ -58,10 +70,11 @@ export class ChatProxyController {
         res.write(Buffer.from(chunk));
       }
     } catch (error: unknown) {
+      this.logStreamError(error);
+
       if (!res.writableEnded && !res.destroyed) {
-        const message = error instanceof Error ? error.message : String(error);
         res.write("event: error\n");
-        res.write(`data: ${JSON.stringify({ error: message })}\n\n`);
+        res.write(`data: ${JSON.stringify({ error: this.toSafeClientMessage(error) })}\n\n`);
       }
     } finally {
       if (!res.writableEnded && !res.destroyed) {
@@ -69,4 +82,21 @@ export class ChatProxyController {
       }
     }
   }
+
+  private toSafeClientMessage(error: unknown): string {
+    if (error instanceof HttpException && error.getStatus() < 500) {
+      return "Chat request was rejected";
+    }
+
+    return "Chat stream failed";
+  }
+
+  private logStreamError(error: unknown): void {
+    if (error instanceof Error) {
+      this.logger.warn(`Chat stream failed: ${error.message}`);
+      return;
+    }
+
+    this.logger.warn(`Chat stream failed: ${String(error)}`);
+  }
 }