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}