feat(inworld tts): add delivery_mode parameter for inworld-tts-2

[chrisackermann]

Summary

• Adds delivery_mode to the Inworld TTS plugin, exposing the TTS-2-only
  deliveryMode output-variation control. Accepts "DELIVERY_MODE_UNSPECIFIED",
  "STABLE", "BALANCED", or "CREATIVE" per the Inworld REST API reference.
• Wired through both transport paths: the WebSocket streaming create packet
  (SynthesizeStream via _InworldConnection._send_loop) and the HTTP
  streaming request body (ChunkedStream._run). Mirrors the existing
  text_normalization / timestamp_type plumbing.
• Surfaced on TTS.__init__, TTS.update_options, and _TTSOptions, with
  the same _validate_str_param validation used by the sibling enum params.

Why

Inworld TTS-2 introduced deliveryMode and silently ignores temperature
on that model. Previously the plugin gave TTS-2 users no way to steer output
variation. Defaults are unchanged — delivery_mode is opt-in and absent from
the wire when unset, letting the server default (BALANCED) apply.

No client-side model-gating: Inworld silently ignores the wrong knob on each
model, and gating client-side would require tracking Inworld's per-model field
support over time. The docstring calls out the TTS-2-only semantics.

Test plan

• uv run pytest tests/test_plugin_inworld_tts.py — 9 new unit tests pass
  (option round-trip + validation for all four documented values + rejection
  of unknown values + wire-payload assertions for both WS and HTTP paths,
  including the negative case that deliveryMode is omitted when unset)
• make check — ruff lint, ruff format, mypy clean
• Live-tested against the real Inworld API end-to-end: rendered the same
  line with delivery_mode="STABLE", "BALANCED", and "CREATIVE" against
  inworld-tts-2. All three were accepted (no fallback to EXPRESSIVE needed —
  the REST docs are authoritative, the Inworld Python SDK docstring is stale)
  and produced audibly different cadence/expressiveness.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

[tinalenguyen]

lgtm and tested it, thank you @chrisackermann!