revert(ai-http): drop resilient fetch/RetryAgent layer (#140)

The custom undici RetryAgent + aiFetch transport added for issue #140
did not actually heal mid-stream provider drops: undici's retry path is
a Range-based download-resume that SSE/chat-completions endpoints cannot
satisfy, so a reset after the first byte only swapped ECONNRESET for a
"server does not support the range header" error. Its only real effect
was reconnecting a poisoned keep-alive socket before the first byte, and
PR #141 on top of it turned the 60s headers timeout into deterministic
~61s failures (plus CONTENT_LENGTH_MISMATCH from retrying a POST body
after a timeout abort). The root cause is the z.ai coding endpoint, not
our transport.

Remove the whole layer and return all AI provider calls to Node's
default global fetch.

- delete integrations/ai/ai-http.ts and its spec
- ai.service.ts: drop the aiFetch import, the AI_BYPASS_RESILIENT_FETCH
  diagnostic toggle, and fetch:aiFetch from every chat/embedding/STT
  factory; raw STT call back to global fetch
- ai-chat.controller.ts: drop the stream-timing START log + startedAt
- ai-chat.service.ts: drop the first-chunk/FINISHED/ERROR timing logs
- .env.example: drop AI_BYPASS_RESILIENT_FETCH

Reverts: 1af5d34a, 7c308728, b7abb7ea, 35fc58ea, d6cd2754, 6efb8656.
Preserved (not part of the rollback): client-disconnect abort, title
generation in onFinish, partial-answer persistence, Safari SSE heartbeat.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/core/ai-chat/ai-chat.controller.ts

@@ -142,9 +142,6 @@ export class AiChatController {
 
     const body = (req.body ?? {}) as AiChatStreamBody;
 
-    // Diagnostic timing baseline for this turn (see START / terminal logs below).
-    const startedAt = Date.now();
-
     // Resolve the agent role for this turn BEFORE hijack: existing chats read it
     // from ai_chats.role_id (authoritative), a new chat from body.roleId. The
     // role drives both the persona and the optional model override below.
@@ -170,7 +167,7 @@ export class AiChatController {
       // so log it here before aborting the agent loop.
       if (!res.raw.writableEnded) {
         this.logger.warn(
-          `AI chat stream: client disconnected before completion after ${Date.now() - startedAt}ms; aborting turn`,
+          'AI chat stream: client disconnected before completion; aborting turn',
         );
         controller.abort();
       }
@@ -178,10 +175,6 @@ export class AiChatController {
     req.raw.once('close', onClose);
     res.raw.once('finish', () => req.raw.off('close', onClose));
 
-    this.logger.log(
-      `AI chat stream START chat=${body.chatId ?? 'new'} ua="${req.headers['user-agent'] ?? ''}"`,
-    );
-
     // Commit to streaming: hijack so Fastify stops managing the response and
     // the AI SDK can write the UI-message stream directly to the Node socket.
     res.hijack();