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.

[github-actions]

This PR has been closed automatically. It's still a draft, but we close those early so you don't put in more time only to have it closed the moment you mark it ready.

This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3345.

If a maintainer assigns you to #3345, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.