Fix FunctionTool not respecting pydantic Field defaults

[Br1an67]

Description

Fix FunctionTool to properly resolve pydantic Field(default=...) values when calling the wrapped function. Previously, if a function parameter used Field(default="Berlin") and wasn't explicitly provided at call time, the raw FieldInfo object was passed instead of the actual default value.

The fix extracts actual default values from FieldInfo objects during __init__ and applies them as lowest-priority defaults in call() and acall().

Fixes #17324

New Package?

N/A

Version Bump?

N/A — bug fix only.

Type of Change

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

How Has This Been Tested?

Added test_function_tool_field_default that creates a function with Field(default="Berlin"), wraps it in FunctionTool, and verifies:

1. Calling without args uses the Field default
2. Calling with an explicit arg overrides the default

All 20 existing tests continue to pass.

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes