Feat/proxy header improvements (#538)

* fix(proxy): improve header handling for Claude API compatibility

- Streamline header blacklist by removing overly aggressive filtering
  (browser-specific headers like sec-fetch-*, accept-language)
- Ensure anthropic-beta header always includes 'claude-code-20250219'
  marker required by upstream services for request validation
- Centralize anthropic-version header handling in forwarder to prevent
  duplicate headers across different auth strategies
- Add ?beta=true query parameter to /v1/messages endpoint for
  compatibility with certain upstream services (e.g., DuckCoding)
- Remove redundant anthropic-version from ClaudeAdapter auth headers
  as it's now managed exclusively by the forwarder

This improves proxy reliability with various Claude API endpoints
and third-party services that have specific header requirements.

* style(services): use inline format arguments in format strings

Apply Rust 1.58+ format string syntax across provider and skill
services. This replaces format!("msg {}", var) with format!("msg {var}")
for improved readability and consistency with modern Rust idioms.

Changed files:
- services/provider/mod.rs: 1 format string
- services/skill.rs: 10 format strings (error messages, log statements)

No functional changes, purely stylistic improvement.

* fix(proxy): restrict Anthropic headers to Claude adapter only

- Move anthropic-beta and anthropic-version header handling inside
  Claude-specific condition to avoid sending unnecessary headers
  to Codex and Gemini APIs
- Update test cases to reflect ?beta=true query parameter behavior
- Add edge case tests for non-messages endpoints and existing queries

src-tauri/src/services/provider/mod.rs

@@ -374,7 +374,7 @@ impl ProviderService {
         let providers = state.db.get_all_providers(app_type.as_str())?;
         let provider = providers
             .get(&current_id)
-            .ok_or_else(|| AppError::Message(format!("Provider {} not found", current_id)))?;
+            .ok_or_else(|| AppError::Message(format!("Provider {current_id} not found")))?;
 
         match app_type {
             AppType::Claude => Self::extract_claude_common_config(&provider.settings_config),