refactor: use Flaps DeleteApp for app deletion

[dangra]

Summary

• switch app deletion paths from API client DeleteApp to Flaps DeleteApp
• update builder cleanup paths to use Flaps for deleting builder apps
• make the mock Flaps client DeleteApp nil-safe for tests

Testing

• go test ./...
• golangci-lint run

[Copilot]

Pull request overview

Refactors application deletion to use the Flaps client’s DeleteApp instead of the API client, aligning destructive app operations with the Machines/Flaps pathway and updating related builder cleanup logic.

Changes:

• Switch apps destroy to delete apps via flapsutil.ClientFromContext(ctx).DeleteApp.
• Update remote builder cleanup paths to delete builder apps via Flaps.
• Make the mock Flaps client’s DeleteApp handler nil-safe to avoid test panics when unset.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/mock/flaps_client.go	Makes DeleteApp a no-op when DeleteAppFunc isn’t provided, improving test robustness.
internal/command/apps/destroy.go	Uses Flaps DeleteApp for app destruction while retaining API client usage for lookups/add-on cleanup.
internal/build/imgsrc/ensure_builder.go	Switches builder-app deletion/cleanup to Flaps DeleteApp, including error-path cleanup.

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