fix(actions): explain why a blocked or waiting job has not started (#38476)

When an Actions job is blocked or waiting, the job view only shows the
generic **Blocked** / **Waiting** label. Users have no way to tell *why*
a job is stuck — whether it's waiting on dependencies, waiting for a
runner that doesn't exist, waiting for a runner whose labels don't
match, or simply queued behind busy runners.

## Change

The current-job detail line now surfaces the actual cause:

- **Blocked** → lists the dependency jobs (`needs`) that haven't
finished yet, e.g. *"Waiting for the following jobs to complete:
build."*
- **Waiting**, no online runner → *"No runner is online to pick up this
job."*
- **Waiting**, online runners but none match `runs-on` → *"No matching
online runner with label: X"* (reuses the existing string)
- **Waiting**, a matching runner exists but hasn't claimed the job →
*"Waiting for a matching runner to become available."*

The runner lookup reuses the same available-online-runner query the run
list already performs, and only runs while a selected job is actually
pending. Dependency resolution is scoped to the same parent job and
treats matrix expansions of a `need` as pending until all of them
complete.

routers/web/repo/actions/view_test.go

@@ -138,3 +138,48 @@ func TestConvertToViewModelCancellingTaskDoesNotRenderRunningSteps(t *testing.T)
 	}
 	assert.Equal(t, expectedViewJobs, viewJobSteps)
 }
+
+func TestPendingNeeds(t *testing.T) {
+	current := &actions_model.ActionRunJob{JobID: "deploy", Needs: []string{"build", "test"}}
+	jobs := []*actions_model.ActionRunJob{
+		current,
+		{JobID: "build", Status: actions_model.StatusSuccess},
+		{JobID: "test", Status: actions_model.StatusRunning},
+	}
+	// "test" is not done yet, "build" succeeded, so only "test" blocks.
+	assert.Equal(t, []string{"test"}, pendingNeeds(current, jobs))
+
+	t.Run("all needs done", func(t *testing.T) {
+		done := []*actions_model.ActionRunJob{
+			current,
+			{JobID: "build", Status: actions_model.StatusSuccess},
+			{JobID: "test", Status: actions_model.StatusSkipped},
+		}
+		assert.Empty(t, pendingNeeds(current, done))
+	})
+
+	t.Run("matrix expansion all required", func(t *testing.T) {
+		matrix := []*actions_model.ActionRunJob{
+			current,
+			{JobID: "build", Status: actions_model.StatusSuccess},
+			{JobID: "build", Status: actions_model.StatusRunning},
+			{JobID: "test", Status: actions_model.StatusSuccess},
+		}
+		assert.Equal(t, []string{"build"}, pendingNeeds(current, matrix))
+	})
+
+	t.Run("unresolved need treated as pending", func(t *testing.T) {
+		missing := []*actions_model.ActionRunJob{current}
+		assert.Equal(t, []string{"build", "test"}, pendingNeeds(current, missing))
+	})
+
+	t.Run("parent job scope", func(t *testing.T) {
+		// a same-named job under a different parent must not satisfy the need
+		scoped := &actions_model.ActionRunJob{JobID: "deploy", Needs: []string{"build"}, ParentJobID: 5}
+		jobs := []*actions_model.ActionRunJob{
+			scoped,
+			{JobID: "build", Status: actions_model.StatusSuccess, ParentJobID: 0},
+		}
+		assert.Equal(t, []string{"build"}, pendingNeeds(scoped, jobs))
+	})
+}