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/core/tests/openai/test_assistant_provider.py

@@ -131,9 +131,15 @@ class TestOpenAIAssistantProviderInit:
         """Test initialization fails without API key when settings return None."""
         from unittest.mock import patch
 
-        # Mock OpenAISettings to return None for api_key
-        with patch("agent_framework.openai._assistant_provider.OpenAISettings") as mock_settings:
-            mock_settings.return_value.api_key = None
+        # Mock load_settings to return a dict with None for api_key
+        with patch("agent_framework.openai._assistant_provider.load_settings") as mock_load:
+            mock_load.return_value = {
+                "api_key": None,
+                "org_id": None,
+                "base_url": None,
+                "chat_model_id": None,
+                "responses_model_id": None,
+            }
 
             with pytest.raises(ServiceInitializationError) as exc_info:
                 OpenAIAssistantProvider()

python/packages/core/tests/openai/test_openai_assistants_client.py

@@ -146,7 +146,7 @@ def test_init_auto_create_client(
 def test_init_validation_fail() -> None:
     """Test OpenAIAssistantsClient initialization with validation failure."""
     with pytest.raises(ServiceInitializationError):
-        # Force failure by providing invalid model ID type - this should cause validation to fail
+        # Force failure by providing invalid model ID type
         OpenAIAssistantsClient(model_id=123, api_key="valid-key")  # type: ignore