fix: override input with json.loads only when output value is valid

[omerc7]

Tool input values in pre_parse_json should only be transformed or cast after the output of json.loads has been validated via Pydantic.

Motivation and Context

The input to the tool may undergo slight transformations in func_metadata.py::pre_parse_json.
For example, a field expected to be a string might be mistakenly cast to a different type. If the input arguments are {"field_name": "5"}, pre_parse_json will incorrectly cast the string "5" to the integer 5, resulting in an error when the field is expected to remain a string.

How Has This Been Tested?

Added num_annotated_with_str to test_func_metadata.py
Tested on local MCP server

Breaking Changes

No

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

Additional context

[ihrpr]

Thank you for contributing to MCP Python SDK!

looks like typecheck and tests are failing

[ihrpr]

Closing this PR as it's stale now. Please re-open when changes are applied and this change is still needed.