fix: populate required fields in FunctionDeclaration json_schema fallback

[giulio-leone]

Summary

Fixes #4798 — required fields lost in FunctionDeclaration when the parameters_json_schema fallback path is used.

Root Cause

from_function_with_options() has two code paths for building function declarations:

1. Primary path (parameters_properties): calls _get_required_fields() ✅
2. Fallback path (parameters_json_schema): triggered when _parse_schema_from_parameter raises ValueError for complex union types (e.g. list[str] | None) — never called _get_required_fields() ❌

Additionally, the fallback path didn't propagate default values from inspect.Parameter to the generated Schema, so _get_required_fields() (which checks schema.default is None) would treat defaulted parameters as required.

Impact

The LLM sees all parameters as optional and may omit required ones. Observed in production: Gemini Flash omitted the mandatory query parameter when calling a tool, because the schema had no required field.

Fix

Three changes to the elif parameters_json_schema branch in from_function_with_options():

Change	Purpose
Added _get_required_fields() call	Populates required field (mirrors primary path)
Propagate non-None defaults to schema.default	Ensures _get_required_fields() correctly excludes defaulted params
Set schema.nullable=True for None-default params	Ensures _get_required_fields() correctly excludes nullable params

Test

Added regression test test_required_fields_set_in_json_schema_fallback that verifies:

• query (no default) → required ✅
• mode (default='default') → not required ✅
• tags: list[str] | None = None → not required ✅

Full test suite: 4726 passed, 0 failures

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