Improve fleet scope validation for software title lookups (#48034)

## Summary

Ensures that `SoftwareTitleByID` validates fleet scope for all non-nil
`team_id` values, including zero. Previously the scope check was only
applied when `team_id > 0`.

## Reproduction

Added a unit test (`TestSoftwareTitleByIDTeamIDZero`) that sets up a
fleet-scoped user on fleet 1, then calls `SoftwareTitleByID` with
`team_id=0`. Before this change, the call succeeded. After, it correctly
returns 403.

Also confirmed that a global admin calling with `team_id=0` still
succeeds, and that all existing `TestServiceSoftwareTitlesAuth` subtests
continue to pass.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements), JS
inline code is prevented especially for url redirects, and untrusted
data interpolated into shell scripts/commands is validated against shell
metacharacters.

## Testing

- [x] Added/updated automated tests

- [x] QA'd all new/changed functionality manually

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Improvements**
* Enhanced fleet scope validation for software title lookups, including
correct handling when a team scope value is set to `0`.

* **Tests**
* Added unit test coverage for software title retrieval authorization
behavior when the team scope value is `0`.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

changes/software-title-scope-validation

@@ -0,0 +1 @@
+- Improved fleet scope validation for software title lookups.

server/service/software_titles.go

@@ -143,17 +143,19 @@ func (svc *Service) SoftwareTitleByID(ctx context.Context, id uint, teamID *uint
 		return nil, err
 	}
 
-	if teamID != nil && *teamID != 0 {
-		// This auth check ensures we return 403 if the user doesn't have access to the team
+	if teamID != nil {
+		// Verify the caller has permission for the requested scope (team or global).
 		if err := svc.authz.Authorize(ctx, &fleet.AuthzSoftwareInventory{TeamID: teamID}, fleet.ActionRead); err != nil {
 			return nil, err
 		}
-		exists, err := svc.ds.TeamExists(ctx, *teamID)
-		if err != nil {
-			return nil, ctxerr.Wrap(ctx, err, "checking if team exists")
-		} else if !exists {
-			return nil, fleet.NewInvalidArgumentError("team_id", fmt.Sprintf("fleet %d does not exist", *teamID)).
-				WithStatus(http.StatusNotFound)
+		if *teamID != 0 {
+			exists, err := svc.ds.TeamExists(ctx, *teamID)
+			if err != nil {
+				return nil, ctxerr.Wrap(ctx, err, "checking if team exists")
+			} else if !exists {
+				return nil, fleet.NewInvalidArgumentError("team_id/fleet_id", fmt.Sprintf("fleet %d does not exist", *teamID)).
+					WithStatus(http.StatusNotFound)
+			}
 		}
 	}
 

server/service/software_titles_test.go

@@ -272,3 +272,38 @@ func TestSoftwareNameUpdate(t *testing.T) {
 	require.NoError(t, err)
 	require.True(t, ds.UpdateSoftwareTitleNameFuncInvoked)
 }
+
+func TestSoftwareTitleByIDTeamIDZero(t *testing.T) {
+	ds := new(mock.Store)
+	ds.SoftwareTitleByIDFunc = func(ctx context.Context, id uint, teamID *uint, tmFilter fleet.TeamFilter) (*fleet.SoftwareTitle, error) {
+		return &fleet.SoftwareTitle{BundleIdentifier: new("com.example.app")}, nil
+	}
+	ds.TeamExistsFunc = func(ctx context.Context, teamID uint) (bool, error) { return true, nil }
+
+	svc, ctx := newTestService(t, ds, nil, nil)
+
+	teamIDZero := new(uint) // *uint pointing to 0
+
+	// Team-scoped user on team 1 should not be able to access software with team_id=0
+	teamUser := &fleet.User{
+		ID: 1,
+		Teams: []fleet.UserTeam{{
+			Team: fleet.Team{ID: 1},
+			Role: fleet.RoleAdmin,
+		}},
+	}
+	ctx = viewer.NewContext(ctx, viewer.Viewer{User: teamUser})
+
+	_, err := svc.SoftwareTitleByID(ctx, 1, teamIDZero)
+	checkAuthErr(t, true, err)
+
+	// Global admin should still be able to access software with team_id=0
+	globalAdmin := &fleet.User{
+		ID:         2,
+		GlobalRole: new(fleet.RoleAdmin),
+	}
+	adminCtx := viewer.NewContext(ctx, viewer.Viewer{User: globalAdmin})
+
+	_, err = svc.SoftwareTitleByID(adminCtx, 1, teamIDZero)
+	checkAuthErr(t, false, err)
+}