feat: add cross-language session message search helpers

[Navanit-git]

Summary

• add client-side session message search helpers for Node.js, Python, Go, .NET, and Java
• search only persisted user.message and assistant.message content returned by the existing history APIs
• support event-type filtering, case sensitivity, and literal/regular-expression matching using idiomatic APIs per language
• add focused Node.js, Python, and Go coverage

Coordination

Relates to #2376.

This draft intentionally overlaps with #2378. That PR currently provides a Node.js-first implementation that recursively searches string values across event data; this draft proposes a narrower conversational-message contract and demonstrates cross-language API parity. I am opening it separately so the alternative semantics and implementation work are visible, and I am happy to coordinate or adjust based on maintainer direction.

Validation

• Python focused tests: uv run pytest test_search_messages.py (3 passed)
• Python Ruff checks: passed
• VS Code diagnostics for all touched files: clean
• Node.js tests: not run because dependency installation was blocked by local npm certificate/cache failures
• Go and .NET checks: not run because those toolchains are unavailable locally
• Java build: not run because the Maven wrapper download was blocked locally

[Navanit-git]

@MackinnonBuck Could you please review this and advise on the API direction, particularly the narrower conversational-message search semantics and cross-language shape?
cc @stephentoub @devm33