fix: skip outputSchema validation when CallToolResult.is_error=True

[RanjithHiremath]

Resolves #2429.

When a tool function with an output_schema returns CallToolResult(is_error=True), convert_result() was unconditionally calling output_model.model_validate(result.structured_content). For an error result, structured_content is None, so model_validate(None) raises a Pydantic ValidationError that replaces the tool's actual error message before it reaches the client.

Skipping schema validation when is_error is true matches the resolution that landed in typescript-sdk#655.

Motivation and Context

Closes #2429.

How Has This Been Tested?

Added test_tool_call_result_with_is_error_skips_output_schema_validation in tests/server/mcpserver/test_func_metadata.py. The test registers a function with an inferred output schema, returns CallToolResult(content=[], is_error=True), and asserts convert_result() returns the result unchanged. Without the fix the test fails with pydantic.ValidationError (because model_validate(None) runs against a model with required fields); with the fix it passes.

Verified locally:

• uv run --frozen ruff check src tests — clean
• uv run --frozen ruff format --check src tests — clean
• uv run --frozen pyright — 0 errors
• ./scripts/test — 100.00% coverage, strict-no-cover passes

Breaking Changes

None.

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 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 (no doc change needed for a behavioral fix)

Additional context

While reviewing, I noticed src/mcp/client/session.py:336 has the same bug pattern client-side: it raises RuntimeError("Tool ... has an output schema but did not return structured content") without first checking result.is_error. I'm scoping that out of this PR (different surface, separate code path) — happy to file a follow-up issue once this is in.

A v1.x backport will follow as a separate PR after this lands (src/mcp/server/fastmcp/utilities/func_metadata.py:115, same change with the v1.x camelCase fields).