fix: address PR review feedback from gemini-code-assist

- Fix parse_custom_headers to preserve commas in values by splitting
  only on commas followed by a header-name:colon pattern (manual scan
  instead of simple split(','))
- Use matches! macro for backend exclusion check in app.rs
- Merge SSE metadata extraction into single pass (was iterating twice)
- Replace fragile substring-based context_length with explicit match
  on known Gemini model IDs via gemini_context_length()
- Add missing models to regression test (8 models, not 5)

tests/gemini_oauth_regression.rs

@@ -62,12 +62,15 @@ fn test_regression_preview_false_positive_fix() {
 }
 
 /// Regression: model list consistency.
-/// Wizard, list_models(), and LLM_PROVIDERS.md all return the same 5 models.
+/// Wizard, list_models(), and LLM_PROVIDERS.md all return the same 8 models.
 #[test]
 fn test_regression_standardized_model_list() {
     let expected_models = [
         "gemini-3.1-pro-preview",
+        "gemini-3.1-pro-preview-customtools",
+        "gemini-3-pro-preview",
         "gemini-3-flash-preview",
+        "gemini-3.1-flash-lite-preview",
         "gemini-2.5-pro",
         "gemini-2.5-flash",
         "gemini-2.5-flash-lite",