feat(client): add validate_output flag to call_tool to skip output-schema revalidation

[RahilOp]

Fixes #2626.

This PR adds a validate_output: bool = True keyword argument to both ClientSession.call_tool() and the high-level Client.call_tool(). When set to False, the client skips revalidating the returned structured content against the tool's declared output schema, allowing callers to work with servers whose output schemas are incomplete or out of sync with the actual result.

Changes:

• Added validate_output to all ClientSession.call_tool() overloads and the implementation.
• Added validate_output to Client.call_tool() and threaded it through the InputRequired retry loop and the claimed-result resolver path.
• Added regression test test_call_tool_validate_output_false_skips_validation.

Verified:

• uv run --frozen pytest tests/client/test_output_schema_validation.py -xvs passes (6/6).
• uv run --frozen ruff format and ruff check clean.
• uv run --frozen pyright src/mcp/client/session.py src/mcp/client/client.py tests/client/test_output_schema_validation.py reports 0 errors.

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[github-actions]

Thanks for the contribution. This repository only keeps pull requests open when they're linked to an issue that a maintainer has assigned to the author — CONTRIBUTING.md explains why and how we work. This PR has been closed for now because you aren't currently assigned to #2626.

If a maintainer would like this change as a PR from you, they'll assign you to #2626 and this PR will reopen automatically — there's nothing more you need to do. (If you opened the issue, this PR already shows up on its timeline.)

There's no need to open a new PR — this one will be reopened. While it's closed, please push any updates as new commits rather than force-pushing, since GitHub can't reopen a PR whose branch has been rewritten.

Maintainers: reopening this PR, removing the missing-issue-link label, or adding bypass-issue-check bypasses the check.