fix(signals): stop the inbox diff failure toast, keep the real reason

frontend/src/initKea.ts

@@ -51,6 +51,7 @@ const ERROR_FILTER_ALLOW_LIST = [
     'loadSelfDrivingEvaluationReports', // The self-driving eval table renders its own retry state
     'loadToolDataEvents',
     'loadInstallRequests', // Polled in the background on Settings → Integrations; the banner just stays hidden
+    'loadReportDiff', // The Inbox report detail's diff panel renders its own error state
     'loadPrChecks', // Polled in the Inbox report detail; the CI checks section renders its own error state
     'loadPrComments', // The Inbox report detail's PR comments section renders its own error state
     'loadMonitoringSnapshot', // The managed warehouse Monitoring tab renders its own retry state

products/signals/frontend/inbox/logics/inboxReportDetailLogic.test.ts

@@ -161,6 +161,42 @@ describe('inboxReportDetailLogic', () => {
         })
     })
 
+    describe('diff fetching', () => {
+        let logic: ReturnType<typeof inboxReportDetailLogic.build>
+
+        beforeEach(() => {
+            useMocks({
+                get: {
+                    '/api/projects/:team_id/signals/reports/:id/artefacts/': { results: [] },
+                    '/api/projects/:team_id/signals/reports/:id/signals/': [],
+                    '/api/projects/:team_id/signals/reports/available_reviewers/': [],
+                    '/api/projects/:team_id/signals/reports/:id/artefacts/:artefact_id/diff/': () => [
+                        404,
+                        { error: "Branch 'fix-checkout' was not found on GitHub." },
+                    ],
+                },
+            })
+            initKeaTests()
+            silenceKeaLoadersErrors()
+            logic = inboxReportDetailLogic({ reportId: REPORT.id, report: REPORT })
+            logic.mount()
+        })
+
+        afterEach(() => {
+            logic.unmount()
+            resumeKeaLoadersErrors()
+        })
+
+        // The endpoint returns its reason under `error`, not DRF's `detail`, so a reducer that reads
+        // the wrong key falls back to guessing at a cause the response already stated.
+        it('surfaces what the endpoint says it could not find', async () => {
+            logic.actions.loadReportDiff({ artefactId: 'artefact-1' })
+            await expectLogic(logic).toFinishAllListeners()
+
+            expect(logic.values.reportDiffError).toBe("Branch 'fix-checkout' was not found on GitHub.")
+        })
+    })
+
     describe('report task polling', () => {
         let logic: ReturnType<typeof inboxReportDetailLogic.build>
         let artefactRequests: number

products/signals/frontend/inbox/logics/inboxReportDetailLogic.ts

@@ -851,15 +851,18 @@ export const inboxReportDetailLogic = kea<inboxReportDetailLogicType>([
                 setFeedbackNoteSubmitting: (_, { submitting }) => submitting,
             },
         ],
-        // Human-readable diff-load failure (kea-loaders only exposes a boolean loading flag). A failed
-        // compare usually means the branch was merged, deleted, or force-rewritten away.
+        // Human-readable diff-load failure (kea-loaders only exposes a boolean loading flag). The
+        // endpoint says which branch or repository GitHub could not find, so prefer its message and
+        // keep the guess for a failure that carried none (a network drop, a gateway error page).
         reportDiffError: [
             null as string | null,
             {
                 loadReportDiff: () => null,
                 loadReportDiffSuccess: () => null,
-                loadReportDiffFailure: () =>
-                    "Couldn't load the diff. The branch may have been merged, deleted, or rewritten.",
+                loadReportDiffFailure: (_state: string | null, { errorObject }: { errorObject?: any }) =>
+                    typeof errorObject?.data?.error === 'string' && errorObject.data.error
+                        ? errorObject.data.error
+                        : "Couldn't load the diff. The branch may have been merged, deleted, or rewritten.",
             },
         ],
         // The commit artefact the current `reportDiff` was loaded for, so the artefact poll re-fetches