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/bedrock/tests/test_bedrock_settings.py

@@ -11,6 +11,7 @@ from agent_framework import (
     FunctionTool,
     Message,
 )
+from agent_framework._settings import load_settings
 from pydantic import BaseModel
 
 from agent_framework_bedrock._chat_client import BedrockChatClient, BedrockSettings
@@ -33,9 +34,9 @@ def _dummy_weather(location: str) -> str:  # pragma: no cover - helper
 def test_settings_load_from_environment(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("BEDROCK_REGION", "us-west-2")
     monkeypatch.setenv("BEDROCK_CHAT_MODEL_ID", "anthropic.claude-v2")
-    settings = BedrockSettings()
-    assert settings.region == "us-west-2"
-    assert settings.chat_model_id == "anthropic.claude-v2"
+    settings = load_settings(BedrockSettings, env_prefix="BEDROCK_")
+    assert settings["region"] == "us-west-2"
+    assert settings["chat_model_id"] == "anthropic.claude-v2"
 
 
 def test_build_request_includes_tool_config() -> None: