fix: don't JSON pre-parse string args whose annotation only accepts str or None

[akminx]

Motivation and Context

Fixes #3055.

FuncMetadata.pre_parse_json decides whether to json.loads a string argument with field_info.annotation is not str. That treats str | None like a non-string annotation, so a valid string value that happens to parse as a JSON object/array (e.g. a JSON-serialized template body) is silently replaced with a dict/list and then fails validation of the argument model with Input should be a valid string.

How Has This Been Tested?

Added test_optional_str_not_parsed_as_json, which fails on main and passes with this change. The full test_func_metadata.py suite passes (43 tests), and ruff check / ruff format are clean.

Implementation notes

The issue suggested skipping pre-parsing for any union that includes str, but that would change the documented behavior covered by test_str_vs_list_str: for str | list[str], a stringified JSON array is still expected to parse into a list (the Claude Desktop workaround this hook exists for). So the new check is narrower: pre-parsing is skipped only when every union member is str or None — in that case a plain string is already fully valid and parsing could only corrupt it. Unions with non-str members keep the existing behavior, and Annotated members are unwrapped before the check.

Breaking Changes

None. Behavior changes only for parameters whose annotation is str/None-only unions, where the previous behavior corrupted valid input.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[akminx]

Closing in favor of #3056, which predates this PR and I hadn't spotted before opening. Left a note there about the Annotated[str, ...] | None spelling this PR also covered.