docs: add TSDoc to LlmSummarizerOptions and LlmSummarizer

[nuthalapativarun]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

Adds TSDoc to core/src/context/summarizers/llm_summarizer.ts, which was missing inline documentation:

• LlmSummarizerOptions interface — adds @property docs for llm and prompt fields
• LlmSummarizer constructor — adds @param options
• LlmSummarizer.summarize() — adds @param events, @returns, and @throws

Testing Plan

Unit Tests:

• All unit tests pass locally.

Manual End-to-End (E2E) Tests:
Documentation-only change; no runtime behavior is altered. Verified with npm run build, npm run lint, and npm test — all pass.

Checklist

• Read CONTRIBUTING.md
• Self-reviewed code
• Commented hard-to-understand areas
• Added tests proving fix/feature works
• Unit tests pass locally
• Manually tested end-to-end
• Dependent changes merged/published

[nuthalapativarun]

Closing in favour of the consolidated PR #413, as requested by @kalenkevich. All changes from this PR are included there.