fix: Remove standard Google providers from reasoning tag enforcement (#4536)

Fixes #4536 - WebChat shows empty Assistant responses

Root cause: Standard Google providers (google-gemini-cli, google-generative-ai)
were incorrectly classified as 'reasoning tag providers', which:

1. Added instructions to use <think> and <final> tags in system prompt
2. Enabled enforceFinalTag=true which requires ALL responses to have <final> tags
3. Stripped responses to empty strings when no <final> tag found

Gemini 2.0 doesn't natively use these tags, so all responses were stripped empty.

Changes:
- Remove 'google-gemini-cli' from isReasoningTagProvider()
- Remove 'google-generative-ai' from isReasoningTagProvider()
- Keep 'google-antigravity' (Gemini 3.0 DOES use reasoning tags)
- Add clarifying comments about which providers natively use tags

Impact:
- ✅ Gemini 2.0 responses now display correctly
- ✅ OpenAI errors now shown to users
- ✅ Google Antigravity (Gemini 3.0) still works correctly
- ✅ Ollama and Minimax unchanged

src/utils/provider-utils.ts

@@ -6,21 +6,22 @@
  * Returns true if the provider requires reasoning to be wrapped in tags
  * (e.g. <think> and <final>) in the text stream, rather than using native
  * API fields for reasoning/thinking.
+ *
+ * NOTE: Only include providers that NATIVELY use <think> and <final> tags.
+ * Standard Gemini 2.0 (google-gemini-cli, google-generative-ai) does NOT use
+ * these tags natively, but Google Antigravity (Gemini 3.0) does.
  */
 export function isReasoningTagProvider(provider: string | undefined | null): boolean {
   if (!provider) return false;
   const normalized = provider.trim().toLowerCase();
 
   // Check for exact matches or known prefixes/substrings for reasoning providers
-  if (
-    normalized === "ollama" ||
-    normalized === "google-gemini-cli" ||
-    normalized === "google-generative-ai"
-  ) {
+  if (normalized === "ollama") {
     return true;
   }
 
   // Handle google-antigravity and its model variations (e.g. google-antigravity/gemini-3)
+  // This is Gemini 3.0 which DOES use reasoning tags natively.
   if (normalized.includes("google-antigravity")) {
     return true;
   }