Add custom automatic context summarization callback

[AzizMuminov]

Summary

• Add an optional should_summarize_callback for application-defined automatic context-summarization policies.
• Expose the callback through LLMAssistantAggregatorParams and preserve existing threshold behavior when it is unset.
• Skip built-in token estimation and threshold evaluation when the callback is configured.

Testing

• uv run pytest tests/test_llm_context_summarizer.py tests/test_context_summarization.py -q
• uv run ruff check src/pipecat/processors/aggregators/llm_context_summarizer.py src/pipecat/processors/aggregators/llm_response_universal.py tests/test_llm_context_summarizer.py

Fixes

• Fixes feat: add should_summarize_callback param to LLMContextSummarizer #4795

[AzizMuminov]

Hi @markbackman — when you have a moment, would you be willing to review this small opt-in API for #4795? It preserves existing automatic threshold behavior unless should_summarize_callback is configured, and the callback path is covered through LLMAssistantAggregator as well as the standalone summarizer. Thanks!