WebChat TTS: add explicit routing/playback logs and user-visible delivery status

Problem

When TTS is triggered from WebChat, operators may see a tool-level success signal but still get no audible output and no downloadable media. Current logs do not clearly show where the WebChat TTS pipeline failed (generation vs routing vs delivery/playback).

Request

Please add explicit structured telemetry (and ideally UI status) for WebChat TTS pipeline stages:

1. tts.generate.started
2. tts.generate.succeeded (provider/model/audio metadata)
3. tts.route.selected (e.g., webchat inline playback, media attachment, local speaker)
4. tts.delivery.succeeded or tts.delivery.failed (with reason/error code)
5. Correlation id linking tool invocation to channel delivery events

Why this helps

This makes it straightforward to distinguish:

• synthesis failures
• channel routing failures
• browser playback policy limitations
• attachment/media delivery failures

Acceptance criteria

• Each WebChat TTS trigger emits a clear route + outcome event in gateway logs.
• Failures include actionable reason codes/messages.
• (Optional but ideal) WebChat UI surfaces final TTS delivery status to the user.

Context

Related bug report with repro/environment details: #57296