feat(ai): add language code support for SpeechConfig

[Lyokone]

Description

Adds languageCode support to SpeechConfig for Firebase AI Live API sessions.

The Gemini Live API supports configuring speech synthesis language through speech_config.language_code for non-native-audio models. FlutterFire previously exposed voiceName but not the matching language code option, so users could not reproduce the same language behavior they get from Vertex AI.

This change:

• adds an optional languageCode parameter to SpeechConfig
• serializes it as language_code in the Live API request payload
• adds a unit test covering voice + language and language-only configurations

Reference: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/live-api/configure-language-voice

Related Issues

• Fixes [firebase_ai]: No support for language_code in Gemini Live API #17459

Checklist

• I added a regression test for the changed behavior.
• I updated public API documentation comments.
• I ran the focused test suite.
• I ran the analyzer.
• This is not a breaking change.

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