[anubhavlal07]

STT.stream(language=...) in livekit-plugins-assemblyai accepted language but ignored it. The stream was always built from the plugin-level language_codes, so a per-stream language never reached AssemblyAI's language steering.

Fix: on the Universal-3 Pro family (_U3_PRO_MODELS, the models that accept language_codes), stream() sets that stream's language_codes from language. It goes through the existing _normalize_language_codes, so "es-ES" becomes ["es"]. The override is on the dataclasses.replace copy the stream already takes, so the provider's options and other streams are unchanged. An empty language means no override.

On other models, which reject language_codes at construction, the override is logged and skipped rather than sent, matching the constructor's restriction.

This is the same gap #8374 fixed for AWS.

Test: two cases.

• test_stream_language_steers_that_stream_only: an override stream gets ["es"], while a default stream and an empty override keep ["en"], and the provider keeps ["en"]. This fails on main.
• test_stream_language_ignored_without_language_steering_support: on universal-streaming-english, nothing is set.

All of tests/test_plugin_assemblyai_stt.py passes (127 tests), and ruff check and ruff format --check are clean.

This change was prepared with the help of an AI assistant (Claude), and I reviewed and tested it.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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