refactor(ai): address PR #176 review — finite-timeout wording, env doc, tests, permanent provider-http module

- Wording: every comment now says the stream timeouts are RAISED to a
  generous-but-finite ~15-min silence timeout, not "disabled (0)" (the stale
  comments contradicted the code, which uses AI_STREAM_TIMEOUT_MS, default
  900000ms).
- Architecture (the load-bearing-temporary trap): the streaming fetch reached
  the chat provider only by riding the "temporary DIAGNOSTIC" telemetry, so
  deleting the telemetry by its own label would silently revert the timeout fix.
  Legitimize it: rename ai-http-diagnostics.ts -> ai-provider-http.ts,
  createDiagnosticFetch -> createInstrumentedFetch, field aiDiagnosticFetch ->
  aiProviderFetch, drop the "temporary" labels, and document the chat transport
  (streaming fetch + instrumentation) as one intentional construct.
- Docs: AI_STREAM_TIMEOUT_MS added to .env.example next to AI_EMBEDDING_TIMEOUT_MS.
- Tests:
  - ai-provider-http.spec: createInstrumentedFetch delegates to the injected
    baseFetch with the same input/init, returns the Response untouched, rethrows
    the error, and defaults to global fetch — covering the baseFetch seam.
  - ai-streaming-fetch.spec: the delayed-server test is now LOAD-BEARING — with
    AI_STREAM_TIMEOUT_MS set below the 1.5s server delay the call actually rejects
    (a lost dispatcher -> global 300s default would NOT), proving the configured
    dispatcher is wired; plus the default-timeout happy path.

server tsc clean; ai-streaming-fetch / ai-provider-http / ai.service / mcp-servers
/ ai-error specs green (41).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/ai/ai.service.ts

@@ -14,8 +14,7 @@ import { AiNotConfiguredException } from './ai-not-configured.exception';
 import { AiEmbeddingNotConfiguredException } from './ai-embedding-not-configured.exception';
 import { AiSttNotConfiguredException } from './ai-stt-not-configured.exception';
 import { describeProviderError } from './ai-error.util';
-// DIAGNOSTIC (provider ECONNRESET investigation) — temporary.
-import { createDiagnosticFetch } from './ai-http-diagnostics';
+import { createInstrumentedFetch } from './ai-provider-http';
 import { createStreamingFetch } from './ai-streaming-fetch';
 import { AiProviderCredentialsRepo } from '@docmost/db/repos/ai-chat/ai-provider-credentials.repo';
 import { SecretBoxService } from '../crypto/secret-box';
@@ -46,12 +45,12 @@ export interface ChatModelOverride {
 export class AiService {
   private readonly logger = new Logger(AiService.name);
 
-  // Provider HTTP fetch for the chat path: a streaming fetch that DISABLES
-  // undici's 300s headers/body timeouts (#175 — long agent turns were severed
-  // mid-stream), wrapped with passive ECONNRESET-investigation telemetry so the
-  // logs observe the exact transport the turn uses. Held for the service
-  // lifetime to reuse the streaming dispatcher's connection pool.
-  private readonly aiDiagnosticFetch = createDiagnosticFetch(
+  // Provider HTTP fetch for the chat path: the streaming fetch — which RAISES
+  // undici's 300s headers/body timeouts to a generous-but-finite silence timeout
+  // so a long agent turn is not severed mid-stream (#175) — wrapped with the
+  // provider-HTTP instrumentation so the logs observe that exact transport. Held
+  // for the service lifetime to reuse the streaming dispatcher's connection pool.
+  private readonly aiProviderFetch = createInstrumentedFetch(
     'AiService:provider-http',
     createStreamingFetch(),
   );
@@ -152,13 +151,12 @@ export class AiService {
         // endpoint. The default callable createOpenAI(...)(model) targets the
         // Responses API (/responses), which OpenAI-compatible gateways
         // (OpenRouter, etc.) reject on multi-turn requests (history with
-        // assistant messages) → 400.
-        // DIAGNOSTIC (provider ECONNRESET investigation) — temporary: pass the
-        // passive instrumented fetch (logging only; no behavior change).
+        // assistant messages) → 400. The provider fetch is the instrumented
+        // streaming fetch (finite-but-generous stream timeouts, #175).
         return createOpenAI({
           apiKey,
           baseURL: baseUrl,
-          fetch: this.aiDiagnosticFetch,
+          fetch: this.aiProviderFetch,
         }).chat(chatModel);
       case 'gemini':
         return createGoogleGenerativeAI({ apiKey })(chatModel);