docs: add TSDoc for agent_transfer, tool_filter, context_compactor, content_request, and agent_registry_mcp processors

[nuthalapativarun]

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Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:
AgentTransferLlmRequestProcessor, ToolFilterRequestProcessor, ContextCompactorRequestProcessor, ContentRequestProcessor, and AgentRegistrySingleMCPToolset have no class-level TSDoc and their public methods lack @param/@returns annotations.

Solution:
Add TSDoc to all five classes following the same conventions as the recently merged #381. Each class gets a summary describing what it does; each public method gets @param and @returns tags. The ToolFilterRequestProcessor inline comment is promoted to a proper TSDoc block.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

This is a documentation-only change; no logic was altered.

Manual End-to-End (E2E) Tests:
N/A — docs-only change.

Checklist

• I have read CONTRIBUTING.md
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.