fix(apps): preserve compaction language anchor

[Sehlani042]

Refs #6273

Summary

• add an explicit language-anchor requirement to the default LlmEventSummarizer prompt
• preserve the conversation language and response-language convention when they are evident from the compacted history
• add a regression test covering an English user turn followed by a foreign-language tool response

Why

The issue reports long-lived sessions drifting to another language after repeated event compaction plus large retrieved tool payloads. This is a narrow prompt-contract mitigation: it makes the compaction summary request preserve the conversation language signal instead of producing a language-neutral summary that later turns can overrun.

This does not claim to fully solve every long-context language drift case; it only hardens the default summarizer behavior at the compaction boundary.

Verification

• RED: uv run --extra test pytest tests/unittests/apps/test_llm_event_summarizer.py::TestLlmEventSummarizer::test_default_prompt_preserves_conversation_language_anchor -q failed before the prompt change because the default prompt lacked the language-anchor instruction
• GREEN: uv run --extra test pytest tests/unittests/apps/test_llm_event_summarizer.py::TestLlmEventSummarizer::test_default_prompt_preserves_conversation_language_anchor -q
• uv run --extra test pytest tests/unittests/apps/test_llm_event_summarizer.py -q
• uv run --extra dev ruff check src/google/adk/apps/llm_event_summarizer.py tests/unittests/apps/test_llm_event_summarizer.py
• uv run --extra dev pyink --check src/google/adk/apps/llm_event_summarizer.py tests/unittests/apps/test_llm_event_summarizer.py
• git diff --check

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[rohityan]

Hi @Sehlani042 , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
While this PR cleanly addresses the language drift aspect, another pull request #6329 was submitted shortly after that implements structured CRITICAL INSTRUCTIONS headers to solve both the language drift and the tool grounding loss in a single prompt update.
To ensure we completely cover both aspects of Issue #6273 with structured metadata anchors, we are going to consolidate the resolution into #6329 and close this PR as superseded.

[rohityan]

Closing this as a du