Clarify missing tool_result validation error

[HarperZ9]

Summary

• keep tool_use / tool_result balance validation strict
• split the missing-follow-up case into a clearer ValueError
• add a direct regression test for an assistant tool_use followed by user text instead of tool_result

Related to #2960. This PR does not relax the balance rule; it only makes the diagnostic distinguish a missing tool_result from an ID mismatch when a tool_use is followed by a non-result message.

Tests

• uv run --frozen pytest tests/server/test_validation.py -q
• uv run --frozen ruff check src/mcp/server/validation.py tests/server/test_validation.py
• uv run --frozen ruff format --check src/mcp/server/validation.py tests/server/test_validation.py
• git diff --check

[cubic-dev-ai]

No issues found across 2 files

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[HarperZ9]

Hi maintainers — checking whether this small validation-error clarification is still useful. The branch is currently mergeable, and the latest recorded CI, conformance, and security checks are green. I can rebase or adjust the wording if the validation contract has moved since this was opened. If the scope is no longer desired, please let me know and I can close it.

[maxisbey]

Thanks for the PR, and sorry it sat here without a proper review.

We're closing most of the open PR backlog. v2 is out and changed a lot of the SDK, so many older PRs no longer apply as written, and we're a small team that realistically doesn't have the capacity to work through the rest.

If this still matters to you on v2, the most useful thing you can do is open an issue (or comment on the existing one) with your use case and a repro. Hearing why it matters to you is what we use to decide what to prioritise.

_{AI Disclaimer}