fix(codex-api): treat invalid_prompt as non-retryable (#9400)

**Goal**: Prevent response.failed events with `invalid_prompt` from
being treated as retryable errors so the UI shows the actual error
message instead of continually retrying.

**Before**: Codex would continue to retry despite the prompt being
marked as disallowed
**After**: Codex will stop retrying once prompt is marked disallowed

codex-rs/codex-api/src/error.rs

@@ -25,6 +25,8 @@ pub enum ApiError {
     },
     #[error("rate limit: {0}")]
     RateLimit(String),
+    #[error("invalid request: {message}")]
+    InvalidRequest { message: String },
 }
 
 impl From<RateLimitError> for ApiError {