Python: [BREAKING] Redesign Python exception hierarchy (#4082)

* [BREAKING] Redesign Python exception hierarchy

Replace the flat ServiceException family with domain-scoped branches:
- AgentException (with InvalidAuth, InvalidRequest, InvalidResponse, ContentFilter)
- ChatClientException (same consistent suberrors)
- IntegrationException (same + InitializationError)
- WorkflowException (Runner, Convergence, Checkpoint, Validation, Action, Declarative)
- ContentError (AdditionItemMismatch)
- ToolException / ToolExecutionException (unchanged)
- MiddlewareException / MiddlewareTermination (unchanged)

Key changes:
- All Service* exceptions removed (ServiceException, ServiceInitializationError, etc.)
- AgentExecutionException split into AgentInvalidRequest/ResponseException
- AgentInvocationError removed, split into AgentInvalidRequest/ResponseException
- Workflow exceptions moved from _workflows/_exceptions.py into main exceptions.py
- _workflows/__init__.py emptied; main __init__.py imports directly from submodules
- Purview exceptions re-parented under IntegrationException hierarchy
- Init validation errors use built-in ValueError/TypeError instead of custom exceptions
- CODING_STANDARD.md updated with hierarchy design and rationale

Fixes microsoft/agent-framework#3410

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Clarify ToolException vs ToolExecutionException docstrings

ToolException: base class for all tool-related exceptions (preconditions,
connection/init failures).
ToolExecutionException: runtime call failures (tool call failed, reconnect
failed, MCP errors).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix remaining stale imports from agent_framework._workflows

- azurefunctions: _context.py, _app.py, _serialization.py, test_func_utils.py
  used 'from agent_framework._workflows import X' which broke after
  emptying _workflows/__init__.py; changed to direct submodule imports
- azure-ai-search: test still referenced ServiceInitializationError;
  updated to ValueError to match production code

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/azure-ai/tests/test_provider.py

@@ -6,7 +6,6 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 from agent_framework import Agent, FunctionTool
 from agent_framework._mcp import MCPTool
-from agent_framework.exceptions import ServiceInitializationError
 from azure.ai.projects.aio import AIProjectClient
 from azure.ai.projects.models import (
     AgentReference,
@@ -110,15 +109,13 @@ def test_provider_init_missing_endpoint() -> None:
     with patch("agent_framework_azure_ai._project_provider.load_settings") as mock_load_settings:
         mock_load_settings.return_value = {"project_endpoint": None, "model_deployment_name": "test-model"}
 
-        with pytest.raises(ServiceInitializationError, match="Azure AI project endpoint is required"):
+        with pytest.raises(ValueError, match="Azure AI project endpoint is required"):
             AzureAIProjectAgentProvider(credential=MagicMock())
 
 
 def test_provider_init_missing_credential(azure_ai_unit_test_env: dict[str, str]) -> None:
     """Test AzureAIProjectAgentProvider initialization when credential is missing."""
-    with pytest.raises(
-        ServiceInitializationError, match="Azure credential is required when project_client is not provided"
-    ):
+    with pytest.raises(ValueError, match="Azure credential is required when project_client is not provided"):
         AzureAIProjectAgentProvider(
             project_endpoint=azure_ai_unit_test_env["AZURE_AI_PROJECT_ENDPOINT"],
         )
@@ -208,7 +205,7 @@ async def test_provider_create_agent_missing_model(mock_project_client: MagicMoc
 
         provider = AzureAIProjectAgentProvider(project_client=mock_project_client)
 
-        with pytest.raises(ServiceInitializationError, match="Model deployment name is required"):
+        with pytest.raises(ValueError, match="Model deployment name is required"):
             await provider.create_agent(name="test-agent")