fix(gitea): surface Gitea error body in merge/FF failures

Gitea returns 405 Method Not Allowed when a merge *style* (e.g.
"fast-forward-only", "merge") is disabled in the repo settings —
NOT when the HTTP method is wrong. The previous error only showed
the status code, making this indistinguishable from a routing bug:

  Gitea merge pull request (POST /repos/…/merge) returned HTTP 405

Now the response body's `message` field is extracted and appended:

  Gitea merge pull request (POST /repos/…/merge) returned HTTP 405:
  merge style 'merge' is not allowed for this repository

This applies to both `fast_forward` and `merge_pull` on GiteaClient.
Non-JSON or body-less errors degrade gracefully (empty detail string).

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

src/gitea_client.rs

@@ -125,10 +125,22 @@ impl GithubClient for GiteaClient {
             ._post(url.clone(), Some(&body))
             .await
             .with_context(|| format!("failed to fast-forward via POST {}", url))?;
-        ensure_success(
-            resp.status().as_u16(),
-            &format!("fast-forward (POST {})", url),
-        )
+        let status = resp.status().as_u16();
+        if (200..300).contains(&status) {
+            return Ok(());
+        }
+        let detail = self
+            .inner
+            .body_to_string(resp)
+            .await
+            .map(|b| extract_message(&b))
+            .unwrap_or_default();
+        Err(anyhow!(
+            "Gitea fast-forward (POST {}) returned HTTP {}: {}",
+            url,
+            status,
+            detail
+        ))
     }
 
     async fn merge_pull(
@@ -148,10 +160,22 @@ impl GithubClient for GiteaClient {
             ._post(url.clone(), Some(&body))
             .await
             .with_context(|| format!("failed to send merge request to POST {}", url))?;
-        ensure_success(
-            resp.status().as_u16(),
-            &format!("merge pull request (POST {})", url),
-        )
+        let status = resp.status().as_u16();
+        if (200..300).contains(&status) {
+            return Ok(());
+        }
+        let detail = self
+            .inner
+            .body_to_string(resp)
+            .await
+            .map(|b| extract_message(&b))
+            .unwrap_or_default();
+        Err(anyhow!(
+            "Gitea merge pull request (POST {}) returned HTTP {}: {}",
+            url,
+            status,
+            detail
+        ))
     }
 
     async fn remove_label(
@@ -183,6 +207,15 @@ impl GithubClient for GiteaClient {
     }
 }
 
+/// Parse Gitea's `{ "message": "..." }` JSON envelope, falling back to
+/// the raw body string if it's not JSON.
+fn extract_message(body: &str) -> String {
+    serde_json::from_str::<serde_json::Value>(body)
+        .ok()
+        .and_then(|v| v.get("message").and_then(|m| m.as_str()).map(String::from))
+        .unwrap_or_else(|| body.to_string())
+}
+
 fn ensure_success(status: u16, what: &str) -> Result<()> {
     if (200..300).contains(&status) {
         Ok(())
@@ -466,4 +499,25 @@ mod tests {
         assert_eq!(pr.labels[0].name, "merge-it");
         assert_eq!(pr.labels[1].name, "release");
     }
+
+    #[test]
+    fn extract_message_gets_json_message_field() {
+        let body = r#"{"message":"merge style 'merge' is not allowed for this repository"}"#;
+        assert_eq!(
+            extract_message(body),
+            "merge style 'merge' is not allowed for this repository"
+        );
+    }
+
+    #[test]
+    fn extract_message_falls_back_to_raw_body_for_non_json() {
+        assert_eq!(extract_message("Not Found"), "Not Found");
+    }
+
+    #[test]
+    fn extract_message_falls_back_when_no_message_field() {
+        let body = r#"{"error":"something"}"#;
+        // No `message` key — returns the raw body.
+        assert_eq!(extract_message(body), body);
+    }
 }

tests/wire.rs

@@ -185,6 +185,14 @@ async fn gitea_merge_pull_surfaces_4xx_as_error() {
         "expected URL path in error for diagnostics: {}",
         err
     );
+    // Gitea's response body contains a `message` field with the actual
+    // reason (e.g. "merge style is not allowed"). It must surface in the
+    // error so the user doesn't need to guess.
+    assert!(
+        err.contains("Pull request is not mergeable"),
+        "expected Gitea's error message in output: {}",
+        err
+    );
 }
 
 #[tokio::test]
@@ -254,14 +262,15 @@ async fn gitea_fast_forward_uses_post_on_merge_endpoint_with_fast_forward_only_d
 
 #[tokio::test]
 async fn gitea_fast_forward_surfaces_4xx_with_url_in_error() {
-    // When Gitea rejects the fast-forward (e.g. older instance without
-    // `fast-forward-only`, or a non-fast-forwardable head), the error
-    // must include both the status code and the path so the user can
-    // see what the action actually called.
+    // When Gitea rejects the fast-forward (e.g. the merge style is
+    // disabled in repo settings), the error must include the status code,
+    // the path, AND Gitea's error message from the response body.
     let server = MockServer::start().await;
     Mock::given(method("POST"))
         .and(path("/repos/octo/widget/pulls/7/merge"))
-        .respond_with(ResponseTemplate::new(405))
+        .respond_with(ResponseTemplate::new(405).set_body_json(json!({
+            "message": "merge style 'fast-forward-only' is not allowed for this repository",
+        })))
         .mount(&server)
         .await;
 
@@ -277,6 +286,11 @@ async fn gitea_fast_forward_surfaces_4xx_with_url_in_error() {
         "expected URL path in error for diagnostics: {}",
         err
     );
+    assert!(
+        err.contains("is not allowed for this repository"),
+        "expected Gitea's error message in output: {}",
+        err
+    );
 }
 
 #[tokio::test]