fix(gemini): fail fast on missing API key + surface it in hermes dump (#15133)

Two small fixes triggered by a support report where the user saw a
cryptic 'HTTP 400 - Error 400 (Bad Request)!!1' (Google's GFE HTML
error page, not a real API error) on every gemini-2.5-pro request.

The underlying cause was an empty GOOGLE_API_KEY / GEMINI_API_KEY, but
nothing in our output made that diagnosable:

1. hermes_cli/dump.py: the api_keys section enumerated 23 providers but
   omitted Google entirely, so users had no way to verify from 'hermes
   dump' whether the key was set. Added GOOGLE_API_KEY and GEMINI_API_KEY
   rows.

2. agent/gemini_native_adapter.py: GeminiNativeClient.__init__ accepted
   an empty/whitespace api_key and stamped it into the x-goog-api-key
   header, which made Google's frontend return a generic HTML 400 long
   before the request reached the Generative Language backend. Now we
   raise RuntimeError at construction with an actionable message
   pointing at GOOGLE_API_KEY/GEMINI_API_KEY and aistudio.google.com.

Added a regression test that covers '', '   ', and None.

hermes_cli/dump.py

@@ -267,6 +267,8 @@ def run_dump(args):
         ("ANTHROPIC_API_KEY", "anthropic"),
         ("ANTHROPIC_TOKEN", "anthropic_token"),
         ("NOUS_API_KEY", "nous"),
+        ("GOOGLE_API_KEY", "google/gemini"),
+        ("GEMINI_API_KEY", "gemini"),
         ("GLM_API_KEY", "glm/zai"),
         ("ZAI_API_KEY", "zai"),
         ("KIMI_API_KEY", "kimi"),