docs/test: add TSDoc and unit tests for LLM processors, summarizer, security plugin, a2a utils, and compacted events

[nuthalapativarun]

Summary

Consolidates six separate docs/test PRs (#397, #398, #399, #400, #401, #402) into one as requested by @kalenkevich.

TSDoc additions:

• BasicLlmRequestProcessor, IdentityLlmRequestProcessor, RequestConfirmationLlmRequestProcessor (closes docs: add TSDoc to BasicLlmRequestProcessor, IdentityLlmRequestProcessor, and RequestConfirmationLlmRequestProcessor #397)
• LlmSummarizerOptions, LlmSummarizer (closes docs: add TSDoc to LlmSummarizerOptions and LlmSummarizer #398)
• PolicyCheckResult, ToolCallPolicyContext, BasePolicyEngine, InMemoryPolicyEngine, SecurityPlugin (closes docs: add TSDoc to SecurityPlugin, policy engine interfaces, and InMemoryPolicyEngine #399)
• a2a utility functions — @param and @returns for all public exports (closes docs: add @param and @returns TSDoc to a2a utility functions #401)

Unit tests:

• isCompactedEvent and createCompactedEvent (closes test: add unit tests for isCompactedEvent and createCompactedEvent #400)
• CodeExecutionRequestProcessor and CodeExecutionResponseProcessor (closes test: add unit tests for CodeExecutionRequestProcessor and CodeExecutionResponseProcessor #402)

Test plan

• npm test passes (all new test files included)
• No new lint errors introduced

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