feat(M3-003): implement OpenAI provider adapter for Codex gpt-5.4

Adds OpenAIAdapter implementing IProviderAdapter, replacing the legacy
registerOpenAIProvider() inline registration in ProviderService.

- Creates apps/gateway/src/agent/adapters/openai.adapter.ts with:
  - register(): initialises OpenAI client, registers codex-gpt-5-4 with
    Pi ModelRegistry using openai-completions API; skips gracefully when
    OPENAI_API_KEY is absent
  - listModels(): returns Codex gpt-5.4 ModelInfo (128k context, tools+vision)
  - healthCheck(): lightweight GET /v1/models to verify API key validity
  - createCompletion(): streaming completions via openai SDK
    chat.completions.create({ stream: true }); maps chunks to CompletionEvent
- Installs openai SDK (^6.32.0) as a dependency in apps/gateway
- Registers OpenAIAdapter in ProviderService alongside OllamaAdapter
- Removes legacy registerOpenAIProvider() private method from ProviderService

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

apps/gateway/src/agent/provider.service.ts

@@ -8,7 +8,7 @@ import type {
   ProviderHealth,
   ProviderInfo,
 } from '@mosaic/types';
-import { OllamaAdapter } from './adapters/index.js';
+import { OllamaAdapter, OpenAIAdapter } from './adapters/index.js';
 import type { TestConnectionResultDto } from './provider.dto.js';
 
 /** DI injection token for the provider adapter array. */
@@ -31,15 +31,14 @@ export class ProviderService implements OnModuleInit {
     this.registry = new ModelRegistry(authStorage);
 
     // Build the default set of adapters that rely on the registry
-    this.adapters = [new OllamaAdapter(this.registry)];
+    this.adapters = [new OllamaAdapter(this.registry), new OpenAIAdapter(this.registry)];
 
     // Run all adapter registrations first (Ollama, and any future adapters)
     await this.registerAll();
 
-    // Register API-key providers directly (Anthropic, OpenAI, Z.ai, custom)
-    // These do not yet have dedicated adapter classes (M3-002 through M3-005).
+    // Register API-key providers directly (Anthropic, Z.ai, custom)
+    // OpenAI now has a dedicated adapter class (M3-003).
     this.registerAnthropicProvider();
-    this.registerOpenAIProvider();
     this.registerZaiProvider();
     this.registerCustomProviders();
 
@@ -234,7 +233,7 @@ export class ProviderService implements OnModuleInit {
 
   // ---------------------------------------------------------------------------
   // Private helpers — direct registry registration for providers without adapters yet
-  // (Anthropic, OpenAI, Z.ai will move to adapters in M3-002 through M3-005)
+  // (Anthropic, Z.ai will move to adapters in M3-002, M3-005)
   // ---------------------------------------------------------------------------
 
   private registerAnthropicProvider(): void {
@@ -257,26 +256,6 @@ export class ProviderService implements OnModuleInit {
     this.logger.log('Anthropic provider registered with 3 models');
   }
 
-  private registerOpenAIProvider(): void {
-    const apiKey = process.env['OPENAI_API_KEY'];
-    if (!apiKey) {
-      this.logger.debug('Skipping OpenAI provider registration: OPENAI_API_KEY not set');
-      return;
-    }
-
-    const models = ['gpt-4o', 'gpt-4o-mini', 'o3-mini'].map((id) =>
-      this.cloneBuiltInModel('openai', id),
-    );
-
-    this.registry.registerProvider('openai', {
-      apiKey,
-      baseUrl: 'https://api.openai.com/v1',
-      models,
-    });
-
-    this.logger.log('OpenAI provider registered with 3 models');
-  }
-
   private registerZaiProvider(): void {
     const apiKey = process.env['ZAI_API_KEY'];
     if (!apiKey) {