refactor(proxy): share auth_header_value helper across provider adapters

claude.rs and gemini.rs each defined an identical `hv` closure that wrapped
`HeaderValue::from_str` into a ProxyError::AuthError result, and codex.rs
spelled the same conversion out inline. /simplify reviewers flagged this
as drift-prone copy-paste.

Move the conversion into a single `pub fn auth_header_value` in
providers/adapter.rs and have the three adapters import it locally. Same
error wording everywhere, one place to update if HeaderValue semantics
ever change.

src-tauri/src/proxy/providers/codex.rs

@@ -177,12 +177,11 @@ impl ProviderAdapter for CodexAdapter {
         &self,
         auth: &AuthInfo,
     ) -> Result<Vec<(http::HeaderName, http::HeaderValue)>, ProxyError> {
+        use super::adapter::auth_header_value;
         let bearer = format!("Bearer {}", auth.api_key);
-        let value = http::HeaderValue::from_str(&bearer)
-            .map_err(|e| ProxyError::AuthError(format!("invalid auth header value: {e}")))?;
         Ok(vec![(
             http::HeaderName::from_static("authorization"),
-            value,
+            auth_header_value(&bearer)?,
         )])
     }
 }