feat: Phase 3 — streaming TTS WebSocket & self-hosted distribution credentials

[dg-coreylweathers]

Phase 3 — Streaming TTS WebSocket + self-hosted credentials

Stacked on #165 (Phase 2). Base is feat/phase-2-read-stream-models; GitHub will retarget to main as the stack merges.

What's included

• [Enhancement] Add Text-to-Speech WebSocket streaming support #148 / [Enhancement] Add Text-to-Speech WebSocket streaming support #147 / Text to Speech - Websocket API #95 — Streaming TTS over a WebSocket. Speak::speak_stream() → SpeakStreamBuilder (model / encoding / sample rate); handle() opens the connection → SpeakStreamHandle with speak / flush / clear / close, and receives audio + events. SpeakStreamHandle: futures::Stream<Item = Result<SpeakResponse>>. SpeakResponse (Audio / Metadata / Flushed / Cleared / Warning / Unknown) is #[non_exhaustive]; unknown message types are preserved rather than breaking the stream. The speak feature now pulls the WebSocket deps. Modeled on src/listen/websocket.rs. Example text_to_speech_websocket.
• Self-hosted distribution credentials. Deepgram::self_hosted() → SelfHosted with list / get / create / delete_distribution_credentials + typed response models. Example self_hosted_credentials.

Verification (Rust 1.97 container)

• build / clippy -D warnings / fmt / cargo test --all --all-features (142 tests, incl. TTS-WS protocol + self-hosted deserialization tests) ✅
• per-feature cargo check incl. speak-only (now pulls WS deps) ✅
• cargo semver-checks: no new breaking changes (only the inherited Phase 1 Extra metadata support #130 streaming breaks).
• Live-verified against production: TTS WebSocket connected, emitted Metadata + Flushed, and streamed 219 KB of audio. Self-hosted list returns a correct 403 INSUFFICIENT_PERMISSIONS on accounts without the self-hosted scope (confirms the request path).

Closes #148, #147, #95

[dg-coreylweathers]

Addressed the review's BLOCKING finding: the TTS-WS worker busy-spun (and hung on current-thread runtimes) after input close because it kept selecting on the drained input channel. run_worker now stops selecting on message_rx once input is closed and only awaits server messages (via a shared handle_incoming helper). Verified: re-ran the example under #[tokio::main(flavor = "current_thread")] — it completes cleanly (219KB audio) where it previously would hang. Also documented the send-then-drain contract and the nil-request_id fallback on SpeakStreamHandle. The self-hosted create/get envelope matches list per the API docs (no secret token field in the create response).