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/memory/sqlite.rs

@@ -138,7 +138,11 @@ impl SqliteMemory {
         // First 8 bytes → 16 hex chars, matching previous format length
         format!(
             "{:016x}",
-            u64::from_be_bytes(hash[..8].try_into().expect("SHA-256 always produces >= 8 bytes"))
+            u64::from_be_bytes(
+                hash[..8]
+                    .try_into()
+                    .expect("SHA-256 always produces >= 8 bytes")
+            )
         )
     }