Python: Replace Pydantic Settings with TypedDict + load_settings() (#3843)

* Replace Pydantic Settings with TypedDict + load_settings()

- Remove pydantic-settings dependency, add python-dotenv
- Delete _pydantic.py (AFBaseSettings, HTTPsUrl)
- Add _settings.py with generic load_settings() function, SecretString,
  type coercion, and Required field validation (SettingNotFoundError)
- Convert all 13 settings classes from AFBaseSettings subclasses to
  TypedDict definitions with load_settings() calls
- Update all consumers from attribute access to dict access
- Add 20 unit tests for load_settings() covering basic loading, dotenv,
  SecretString, type coercion, and required field validation
- Update all existing tests for new settings patterns

* Fix mypy type errors from settings conversion

- Fix str | None attribute access in responses_client (walrus operator)
- Fix SecretString | None narrowing in bedrock (type: ignore after guard)
- Convert _context_provider.py attribute access to dict access (missed file)
- Fix endpoint type narrowing in search_provider and context_provider
- Fix purview: str | None .rstrip(), int | None defaults, urlparse bytes

* Address PR review: required_fields param, type validation, fixes

- Move required field validation from TypedDict annotations (Required)
  to a required_fields parameter on load_settings(), enabling runtime
  decisions about which fields are required
- Remove Required imports and restore from __future__ import annotations
  in ollama and foundry_local
- Add _check_override_type() for deterministic ServiceInitializationError
  on invalid override types (e.g. dict passed for str field)
- Fix all multi-exception test catches back to single exception type
- Fix Ollama host=None: use .get() so None is passed through to SDK default
- Fix Purview processor: use explicit is-None checks instead of or operator
- Remove unused BaseModel import from openai/_shared.py
- Add 4 new tests (24 total): required_fields param, type validation

* Fix type validation: allow int for float fields

_check_override_type now permits int values for float-typed fields,
matching Python's standard numeric promotion behavior.

* fix: wrap urlparse arg with str() to fix mypy bytes endswith error

python/packages/foundry_local/tests/test_foundry_local_client.py

@@ -4,8 +4,8 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 from agent_framework import SupportsChatGetResponse
-from agent_framework.exceptions import ServiceInitializationError
-from pydantic import ValidationError
+from agent_framework._settings import load_settings
+from agent_framework.exceptions import ServiceInitializationError, SettingNotFoundError
 
 from agent_framework_foundry_local import FoundryLocalClient
 from agent_framework_foundry_local._foundry_local_client import FoundryLocalSettings
@@ -15,31 +15,43 @@ from agent_framework_foundry_local._foundry_local_client import FoundryLocalSett
 
 def test_foundry_local_settings_init_from_env(foundry_local_unit_test_env: dict[str, str]) -> None:
     """Test FoundryLocalSettings initialization from environment variables."""
-    settings = FoundryLocalSettings(env_file_path="test.env")
+    settings = load_settings(FoundryLocalSettings, env_prefix="FOUNDRY_LOCAL_", env_file_path="test.env")
 
-    assert settings.model_id == foundry_local_unit_test_env["FOUNDRY_LOCAL_MODEL_ID"]
+    assert settings["model_id"] == foundry_local_unit_test_env["FOUNDRY_LOCAL_MODEL_ID"]
 
 
 def test_foundry_local_settings_init_with_explicit_values() -> None:
     """Test FoundryLocalSettings initialization with explicit values."""
-    settings = FoundryLocalSettings(model_id="custom-model-id", env_file_path="test.env")
+    settings = load_settings(
+        FoundryLocalSettings,
+        env_prefix="FOUNDRY_LOCAL_",
+        model_id="custom-model-id",
+        env_file_path="test.env",
+    )
 
-    assert settings.model_id == "custom-model-id"
+    assert settings["model_id"] == "custom-model-id"
 
 
 @pytest.mark.parametrize("exclude_list", [["FOUNDRY_LOCAL_MODEL_ID"]], indirect=True)
 def test_foundry_local_settings_missing_model_id(foundry_local_unit_test_env: dict[str, str]) -> None:
-    """Test FoundryLocalSettings when model_id is missing raises ValidationError."""
-    with pytest.raises(ValidationError):
-        FoundryLocalSettings(env_file_path="test.env")
+    """Test FoundryLocalSettings when model_id is missing raises error."""
+    with pytest.raises(SettingNotFoundError, match="Required setting 'model_id'"):
+        load_settings(
+            FoundryLocalSettings,
+            env_prefix="FOUNDRY_LOCAL_",
+            required_fields=["model_id"],
+            env_file_path="test.env",
+        )
 
 
 def test_foundry_local_settings_explicit_overrides_env(foundry_local_unit_test_env: dict[str, str]) -> None:
     """Test that explicit values override environment variables."""
-    settings = FoundryLocalSettings(model_id="override-model-id", env_file_path="test.env")
+    settings = load_settings(
+        FoundryLocalSettings, env_prefix="FOUNDRY_LOCAL_", model_id="override-model-id", env_file_path="test.env"
+    )
 
-    assert settings.model_id == "override-model-id"
-    assert settings.model_id != foundry_local_unit_test_env["FOUNDRY_LOCAL_MODEL_ID"]
+    assert settings["model_id"] == "override-model-id"
+    assert settings["model_id"] != foundry_local_unit_test_env["FOUNDRY_LOCAL_MODEL_ID"]
 
 
 # Client Initialization Tests