fix(proxy): return Result from get_auth_headers to avoid panic on bad credentials

User-pasted API keys can contain control chars or CR/LF that make
HeaderValue::from_str return Err; the previous unwrap inside every
adapter turned such input into a process-wide panic instead of a request
error. The trait now returns Result<_, ProxyError>; Claude/Codex/Gemini
impls propagate ProxyError::AuthError so the client sees a 401 with the
underlying parse error instead of a crash. Adds a regression test that
pastes a CRLF-containing key and asserts AuthError.

src-tauri/src/proxy/forwarder.rs

@@ -1151,7 +1151,7 @@ impl RequestForwarder {
                 }
             }
 
-            adapter.get_auth_headers(&auth)
+            adapter.get_auth_headers(&auth)?
         } else {
             Vec::new()
         };