fix(validation): only match tool_use/tool_result IDs when last message has tool_result

[syf2211]

Summary

Fix a false positive in validate_tool_use_result_messages() when a previous tool_use block is followed by a plain text response without any tool_result blocks.

Motivation

Per SEP-1577, tool_result blocks must be preceded by a tool_use block, but there is no requirement that tool_use must be followed by tool_result. A plain text user reply after an assistant tool_use is valid.

Fixes #2960

Changes

• Move the tool_use/tool_result ID matching check inside the existing if has_tool_results: block in src/mcp/server/validation.py
• Add regression test reproducing the issue scenario

Tests

uv run pytest tests/server/test_validation.py -v

Result: 16 passed

Also ran:

uv run ruff check src/mcp/server/validation.py tests/server/test_validation.py

Result: All checks passed

Notes

AI assistance was used to implement this fix. The change is minimal (3-line scope move) and aligns with the root cause described in the issue.

Disclosure: This PR was prepared with AI assistance and reviewed before submission.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}