fix: semantic filter lazy-build router + handle missing metadata key

Two bugs from PR #20296 (MCP Semantic Filtering):

1. filter_tools() returned all tools when tool_router was None instead
   of lazily building the router from available_tools. This caused
   top_k to never limit results.

2. async_pre_call_hook() crashed with KeyError when data dict had no
   metadata key, causing the exception handler to return None (no
   filtering applied).

litellm/proxy/_experimental/mcp_server/semantic_tool_filter.py

@@ -172,10 +172,20 @@ class SemanticMCPToolFilter:
         if not query or not query.strip():
             return available_tools
 
-        # Router should be built on startup - if not, something went wrong
+        # Lazy-build router from available_tools if not already initialized
         if self.tool_router is None:
-            verbose_logger.warning("Router not initialized - was build_router_from_mcp_registry() called on startup?")
-            return available_tools
+            verbose_logger.info(
+                "Router not pre-built, building from available_tools on first call"
+            )
+            try:
+                self._build_router(available_tools)
+            except Exception as e:
+                verbose_logger.error(f"Failed to lazy-build router: {e}")
+                return available_tools
+
+            if self.tool_router is None:
+                verbose_logger.warning("Router still None after build attempt")
+                return available_tools
 
         # Run semantic filtering
         try:

litellm/proxy/hooks/mcp_semantic_filter/hook.py

@@ -224,6 +224,8 @@ class SemanticToolFilterHook(CustomLogger):
             tool_names_csv = self._get_tool_names_csv(filtered_tools)
             
             _metadata_variable_name = self._get_metadata_variable_name(data)
+            if _metadata_variable_name not in data:
+                data[_metadata_variable_name] = {}
             data[_metadata_variable_name]["litellm_semantic_filter_stats"] = filter_stats
             data[_metadata_variable_name]["litellm_semantic_filter_tools"] = tool_names_csv