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/tools/browser.rs

@@ -793,20 +793,14 @@ mod tests {
     #[test]
     fn extract_host_handles_ipv6() {
         // IPv6 with brackets (required for URLs with ports)
-        assert_eq!(
-            extract_host("https://[::1]/path").unwrap(),
-            "[::1]"
-        );
+        assert_eq!(extract_host("https://[::1]/path").unwrap(), "[::1]");
         // IPv6 with brackets and port
         assert_eq!(
             extract_host("https://[2001:db8::1]:8080/path").unwrap(),
             "[2001:db8::1]"
         );
         // IPv6 with brackets, trailing slash
-        assert_eq!(
-            extract_host("https://[fe80::1]/").unwrap(),
-            "[fe80::1]"
-        );
+        assert_eq!(extract_host("https://[fe80::1]/").unwrap(), "[fe80::1]");
     }
 
     #[test]