fix: resolve raw language strings through Language enum for proper service conversion

[markbackman]

Summary

• Fixed raw language strings like "de-DE" silently failing when passed to TTS/STT services. Previously, raw strings bypassed the Language enum resolution logic and were sent directly to the provider API (e.g. ElevenLabs expects "de", not "de-DE"), causing no audio output with no error.
• Raw strings are now converted to Language enums first so they go through the same resolve_language() path as enum values, properly resolving regional codes to service-expected formats.
• Unrecognized language strings now log a warning instead of failing silently.
• The fix applies to both TTSService and STTService base classes, covering both init-time and runtime update paths.

Testing

uv run pytest tests/test_service_language.py -v

23 tests covering all permutations of:

• Input types: Language enum (base), Language enum (regional), raw string (base), raw string (regional), unrecognized string, None
• Code paths: init-time settings, runtime _update_settings()
• Service types: TTS and STT

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/pipecat/services/stt_service.py	40.14% <100.00%> (+4.37%)	⬆️
src/pipecat/services/tts_service.py	65.06% <100.00%> (+1.66%)	⬆️

... and 4 files with indirect coverage changes

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[filipi87]

LGTM!