fix(showcase/ms-agent-python): pin transcription baseURL to real OpenAI

The voice route's `GuardedOpenAITranscriptionService` was constructing
`new OpenAI({ apiKey })` without a `baseURL` override. The OpenAI
client falls back to the `OPENAI_BASE_URL` env var, which production
docker/Railway sets to `http://aimock:4010/v1` so LLM completions stay
deterministic. aimock's transcription handler then returned a 502
"Invalid file format" (or a canned "What is the weather in Tokyo?"
fixture on dev), surfacing as "CopilotChat: Transcription failed" on
every mic recording.

Mirrored langgraph-python's voice route: read
`OPENAI_TRANSCRIPTION_BASE_URL` first, fall back to
`https://api.openai.com/v1`. The sample-audio button stays
deterministic (synchronous text injection); the mic now exercises real
Whisper.

Author: AlemTuzlak

showcase/integrations/ms-agent-python/src/app/api/copilotkit-voice/[[...slug]]/route.ts

@@ -31,14 +31,38 @@ const AGENT_URL = process.env.AGENT_URL || "http://localhost:8000";
 // response instead of a tool call that the agent can't summarize.
 const voiceDemoAgent = new HttpAgent({ url: `${AGENT_URL}/voice/` });
 
+/**
+ * Transcription service wrapper that reports a clean, typed auth error when
+ * OPENAI_API_KEY is not configured. When the key is present we delegate to
+ * the real OpenAI-backed service; any upstream Whisper error keeps its
+ * natural categorization.
+ *
+ * Note: We pin `baseURL` to real OpenAI (or `OPENAI_TRANSCRIPTION_BASE_URL`
+ * when explicitly set) instead of falling through to `OPENAI_BASE_URL`. In
+ * local docker / Railway preview environments `OPENAI_BASE_URL` points at
+ * aimock so LLM completions stay deterministic, but aimock has a catchall
+ * `endpoint: "transcription"` fixture that would otherwise intercept every
+ * real mic recording and return the canned "What is the weather in Tokyo?"
+ * phrase regardless of what the user actually said — and on production
+ * aimock's transcription proxy returns a 502 "Invalid file format" before
+ * any phrase reaches the user. The sample-audio button is the deterministic
+ * affordance (synchronous text injection); the mic is the only path that
+ * should exercise real Whisper.
+ *
+ * Mirrors langgraph-python's voice route exactly.
+ */
 class GuardedOpenAITranscriptionService extends TranscriptionService {
   private delegate: TranscriptionServiceOpenAI | null;
 
   constructor() {
     super();
     const apiKey = process.env.OPENAI_API_KEY;
+    const baseURL =
+      process.env.OPENAI_TRANSCRIPTION_BASE_URL ?? "https://api.openai.com/v1";
     this.delegate = apiKey
-      ? new TranscriptionServiceOpenAI({ openai: new OpenAI({ apiKey }) })
+      ? new TranscriptionServiceOpenAI({
+          openai: new OpenAI({ apiKey, baseURL }),
+        })
       : null;
   }