fix(client): validate JSON-null structuredContent against outputSchema

[epistemedeus]

Fixes #3345

SEP-2106 allows structuredContent to be JSON null. validate_tool_result treated structured_content is None as a missing field, so a tool that advertised a null-capable outputSchema and returned "structuredContent": null never reached jsonschema validation.

Pydantic stores both omitted and explicit JSON null as None. This uses model_fields_set as the presence check, matching the TypeScript SDK's === undefined (not null / not falsy) check.

Motivation and Context

Declared outputSchema was not validated against a legal JSON-null structuredContent value. Clients rejected a conforming result as missing instead of schema-checking it. Omitted structuredContent still fails closed.

How Has This Been Tested?

uv run --frozen pytest tests/client/test_session_promotions.py tests/interaction/lowlevel/test_tools.py::test_declared_output_schema_with_no_structured_content_is_rejected_by_the_client tests/interaction/lowlevel/test_tools.py::test_call_tool_structured_content_violating_output_schema_is_rejected_by_the_client -q

24 passed, including new cases for:

• explicit JSON null against {"type": "null"} (accept)
• explicit JSON null against an object schema (schema mismatch, not missing)
• omitted field (existing missing-field error)
• falsy non-null values 0 / false

Breaking Changes

None. Omitted structuredContent still raises the same RuntimeError. Only an explicit JSON null is now schema-validated.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I am assigned to the linked issue (or it is labeled help wanted, or I'm a maintainer)
• I have disclosed any AI assistance and can explain the change in my own words
• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Not assigned on #3345. Opening as a draft per the small-fix-with-tests shape; CONTRIBUTING may auto-close until a maintainer assigns the issue. Reporter of #3345.

AI assistance: researched and implemented with Grok 4.6; I reviewed the spec text, the TypeScript v2 presence check, Pydantic model_fields_set on parsed CallToolResult, and the new tests.

No protocol change. No new schema surface. Does not duplicate the existing jsonschema check; it only stops the presence check from swallowing JSON null before that check runs.