Python: [Bug]: load_settings silently accepts invalid values for non-Optional union fields #8907

Description

_coerce_value (agent_framework/_settings.py) only runs its per-arm union coercion when None is one of the arms. For any other union, e.g. int | float, it falls through to return value. The raw string is returned unchanged, and invalid input is accepted without error.

This contradicts the documented contract in packages/core/AGENTS.md: "Invalid supplied numeric and boolean environment or .env values raise ValueError identifying the field and source".

Suggested fix: handle any Union / types.UnionType origin with the same try-each-arm loop, treating the None arm as optional, and raise when no arm matches.

Code Sample

from agent_framework._settings import _coerce_value

_coerce_value("5", int | None)    # 5      (correct)
_coerce_value("5", int | float)   # '5'    (str, expected 5)
_coerce_value("abc", int | float) # 'abc'  (expected ValueError)

Error Messages / Stack Traces

Package Versions

agent-framework-core 1.19.0 (main @ 853c456)

Python Version

Python 3.11

Additional Context

Found during a code review of main @ 853c456 (2026-09-30); the repro above was run against that commit.