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/onboard/wizard.rs

@@ -1734,9 +1734,8 @@ fn setup_channels() -> Result<ChannelsConfig> {
                     }
                 };
 
-                let nickname: String = Input::new()
-                    .with_prompt("  Bot nickname")
-                    .interact_text()?;
+                let nickname: String =
+                    Input::new().with_prompt("  Bot nickname").interact_text()?;
 
                 if nickname.trim().is_empty() {
                     println!("  {} Skipped — nickname required", style("→").dim());
@@ -1779,7 +1778,9 @@ fn setup_channels() -> Result<ChannelsConfig> {
                 };
 
                 if allowed_users.is_empty() {
-                    print_bullet("⚠️  Empty allowlist — only you can interact. Add nicknames above.");
+                    print_bullet(
+                        "⚠️  Empty allowlist — only you can interact. Add nicknames above.",
+                    );
                 }
 
                 println!();