fix(hook): make gemini runner fail open on empty stdin

Amendment to the rtk-ai#2565 cherry-pick: run_gemini parsed input without the
empty-input guard the other five runners have, so the new stdin timeout
(and plain EOF-without-payload) made it exit nonzero instead of failing
open. Guard empty input with the allow response, and cover it with an
integration test mirroring the claude one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: kylehgc

src/hooks/hook_cmd.rs

@@ -415,7 +415,16 @@ fn copilot_cli_response_from_decision(
 pub fn run_gemini() -> Result<()> {
     let input = read_stdin_limited()?;
 
-    let json: Value = serde_json::from_str(&input).context("Failed to parse hook input as JSON")?;
+    // Fail open on empty input (EOF or stdin timeout), like every other
+    // runner: without this guard the serde parse fails and the hook exits
+    // nonzero, blocking the tool call it gates.
+    let input = input.trim();
+    if input.is_empty() {
+        print_allow();
+        return Ok(());
+    }
+
+    let json: Value = serde_json::from_str(input).context("Failed to parse hook input as JSON")?;
 
     let tool_name = json.get("tool_name").and_then(|v| v.as_str()).unwrap_or("");
 

tests/hook_stdin_timeout_test.rs

@@ -77,3 +77,43 @@ fn claude_hook_fails_open_when_stdin_stays_open_without_payload() {
         thread::sleep(Duration::from_millis(25));
     }
 }
+
+#[test]
+fn gemini_hook_fails_open_when_stdin_stays_open_without_payload() {
+    let mut child = Command::new(env!("CARGO_BIN_EXE_rtk"))
+        .args(["hook", "gemini"])
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .expect("spawn rtk hook gemini");
+
+    let stdin = child.stdin.take().expect("child stdin");
+    let deadline = Instant::now() + Duration::from_secs(3);
+
+    loop {
+        if child.try_wait().expect("poll child").is_some() {
+            drop(stdin);
+            let output = child.wait_with_output().expect("collect hook output");
+            assert!(
+                output.status.success(),
+                "hook should fail open with exit 0, got {:?}, stderr={}",
+                output.status.code(),
+                String::from_utf8_lossy(&output.stderr)
+            );
+            // Gemini's protocol expects a JSON response; fail-open is allow.
+            let response: serde_json::Value =
+                serde_json::from_slice(&output.stdout).expect("gemini JSON response");
+            assert_eq!(response["decision"], "allow");
+            return;
+        }
+
+        if Instant::now() >= deadline {
+            let _ = child.kill();
+            let _ = child.wait();
+            panic!("rtk hook gemini blocked on open stdin without payload");
+        }
+
+        thread::sleep(Duration::from_millis(25));
+    }
+}