fix: validate full sampling tool result history

[Epochex]

Summary

validate_tool_use_result_messages only checked the last message pair. If an earlier tool_result was malformed or did not match the previous tool_use, a later plain message made validation pass and the invalid history could still be sent to the sampling client.

This changes the validator to walk the whole message history and require every tool_use message to be followed by matching tool_result blocks.

Tests

• uv run ruff check .
• uv run ruff format --check .
• uv run pyright --pythonplatform Linux
• uv run pytest -q

[maxisbey]

Thanks for the PR. Per our CONTRIBUTING.md we ask that PRs start from a linked, triaged issue with maintainer buy-in, which this one doesn't have yet. Closing this as part of a wider backlog cleanup around the v2 release. If it's still relevant, feel free to reopen.

_{AI Disclaimer}