refactor(aicore/filtering): collapse public API into filters.py

Address review comment #14 ("can we have all public methods on a
filters.py? Better to read").

Merge _filters.py + _modules.py + _api.py into a single filters.py
(no underscore — signals the public surface). It now owns Severity,
ContentFilter, AzureContentFilter, LlamaGuard38bFilter, InputFiltering,
OutputFiltering, ContentFiltering, set_filtering, disable_filtering.
Tests import directly from .filters; the package __init__ stays a thin
re-export for the flat 'from sap_cloud_sdk.aicore import ...' path.

Internal-only files (_litellm_patch, exceptions) keep their separate
locations; they aren't part of the documented public surface.

Author: lenin-ribeiro

src/sap_cloud_sdk/aicore/filtering/__init__.py

@@ -8,24 +8,26 @@
 
 See :mod:`sap_cloud_sdk.aicore` user guide for the documented public API.
 
-This module is a thin re-export surface; implementations live in
-``_api`` (entry points), ``_filters`` (provider classes + ``Severity``),
-``_modules`` (direction containers), ``_litellm_patch`` (LiteLLM patch),
-and ``exceptions``.
+This module is a thin re-export surface; the public API lives in
+:mod:`.filters`. Internal pieces live in ``_litellm_patch`` (LiteLLM patch
+and ``_install``) and ``exceptions``.
 """
 
 from __future__ import annotations
 
-from ._api import disable_filtering, set_filtering
-from ._filters import (
+from ._litellm_patch import extract_filter_blocked
+from .exceptions import ContentFilteredError, OrchestrationError
+from .filters import (
     AzureContentFilter,
     ContentFilter,
+    ContentFiltering,
+    InputFiltering,
     LlamaGuard38bFilter,
+    OutputFiltering,
     Severity,
+    disable_filtering,
+    set_filtering,
 )
-from ._litellm_patch import extract_filter_blocked
-from ._modules import ContentFiltering, InputFiltering, OutputFiltering
-from .exceptions import ContentFilteredError, OrchestrationError
 
 __all__ = [
     "set_filtering",