fix(models): restore gemini-3-flash-preview to Gemini OAuth picker (#37606)

#37046 swapped gemini-3-flash-preview -> gemini-3.5-flash in the
google-gemini-cli (OAuth/Code Assist) picker on the premise that the
preview slug was renamed. It wasn't. Per gemini-cli's models.ts, Code
Assist serves two distinct flash slugs with different access gates:
gemini-3-flash-preview (PREVIEW_GEMINI_FLASH_MODEL — what subscription/
free-tier OAuth users reach) and gemini-3.5-flash
(DEFAULT_GEMINI_3_5_FLASH_MODEL — GA-channel-gated). The model string is
passed verbatim into the {project, model, ...} envelope sent to
cloudcode-pa.googleapis.com, so non-GA users got a hard error on every
prompt because gemini-3.5-flash 404s for them.

Offer both slugs in the OAuth picker (matching gemini-cli's own /model
list) so non-GA users can select the preview flash that works. The
gemini (API-key), OpenRouter, and Nous lists are untouched —
google/gemini-3.5-flash is a real live model on those surfaces.

hermes_cli/models.py

@@ -241,6 +241,12 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
     "google-gemini-cli": [
         "gemini-3.1-pro-preview",
         "gemini-3-pro-preview",
+        # Code Assist serves two flash slugs with different access gates
+        # (gemini-cli models.ts): gemini-3-flash-preview is the preview flash
+        # that subscription/free-tier OAuth users actually reach, while
+        # gemini-3.5-flash is GA-channel-gated. Offer both so non-GA users
+        # aren't stuck with a slug cloudcode-pa 404s for them.
+        "gemini-3-flash-preview",
         "gemini-3.5-flash",
     ],
     "zai": [