fix(core): preserve primitive validation in JSON Schema unions

[黄耀鑫(Huang Yaoxin) (YaoxinHuang)]

Why are these changes needed?

schema_to_pydantic_model() drops numeric bounds, string constraints, and formats from primitive anyOf/oneOf branches. For example, an integer-or-null field with minimum: 0 accepts -1 after conversion.

Use the existing field-type conversion for string, integer, and number union branches, preserving their validation. Array, reference, enum, and object handling stays the same. Add regression cases for numeric bounds, string lengths/patterns, and email format, including valid boundary values and null.

Related issue number

No existing issue found for primitive union validation. This is separate from #6925, which handled array branches.

Validation

• The 16 new cases fail against the unchanged implementation.
• pytest packages/autogen-core/tests/test_json_to_pydantic.py -q: 91 passed.
• Ruff formatting/lint, Mypy, and Pyright passed for both changed files.
• Full workspace, documentation, and integration checks were not run locally.

Checks

• I've included any doc changes needed (no public API or documentation changes).
• I've added tests corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (upstream CI pending).

This change and its regression tests were prepared with Codex.