fix(providers): use Bearer auth for Gemini CLI OAuth tokens

* fix(providers): use Bearer auth for Gemini CLI OAuth tokens

When credentials come from ~/.gemini/oauth_creds.json (Gemini CLI),
send them as Authorization: Bearer header instead of ?key= query
parameter. API keys from env vars or config continue using ?key=.

Fixes #194

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor(gemini): harden OAuth bearer auth flow and tests

* fix(gemini): granular auth source tracking and review fixes

Build on chumyin's auth model refactor with:
- Expand GeminiAuth to 4 variants (ExplicitKey/EnvGeminiKey/EnvGoogleKey/
  OAuthToken) so auth_source() uses stored discriminant without re-reading
  env vars at call time
- Add is_api_key()/credential() helpers on the enum
- Upgrade expired OAuth token log from debug to warn
- Add tests: provider_rejects_empty_key, auth_source_explicit_key,
  auth_source_none_without_credentials

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply rustfmt to fix CI lint failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: root <root@instance-20220913-1738.vcn09131738.oraclevcn.com>
Co-authored-by: argenis de la rosa <theonlyhennygod@gmail.com>

src/providers/compatible.rs

@@ -452,14 +452,20 @@ mod tests {
     fn chat_completions_url_standard_openai() {
         // Standard OpenAI-compatible providers get /chat/completions appended
         let p = make_provider("openai", "https://api.openai.com/v1", None);
-        assert_eq!(p.chat_completions_url(), "https://api.openai.com/v1/chat/completions");
+        assert_eq!(
+            p.chat_completions_url(),
+            "https://api.openai.com/v1/chat/completions"
+        );
     }
 
     #[test]
     fn chat_completions_url_trailing_slash() {
         // Trailing slash is stripped, then /chat/completions appended
         let p = make_provider("test", "https://api.example.com/v1/", None);
-        assert_eq!(p.chat_completions_url(), "https://api.example.com/v1/chat/completions");
+        assert_eq!(
+            p.chat_completions_url(),
+            "https://api.example.com/v1/chat/completions"
+        );
     }
 
     #[test]
@@ -515,14 +521,20 @@ mod tests {
     fn chat_completions_url_without_v1() {
         // Provider configured without /v1 in base URL
         let p = make_provider("test", "https://api.example.com", None);
-        assert_eq!(p.chat_completions_url(), "https://api.example.com/chat/completions");
+        assert_eq!(
+            p.chat_completions_url(),
+            "https://api.example.com/chat/completions"
+        );
     }
 
     #[test]
     fn chat_completions_url_base_with_v1() {
         // Provider configured with /v1 in base URL
         let p = make_provider("test", "https://api.example.com/v1", None);
-        assert_eq!(p.chat_completions_url(), "https://api.example.com/v1/chat/completions");
+        assert_eq!(
+            p.chat_completions_url(),
+            "https://api.example.com/v1/chat/completions"
+        );
     }
 
     // ══════════════════════════════════════════════════════════