chore: Add a test on viewing and interacting with a charm in shell integration

[jsantell]

Summary by cubic

Added an integration test for viewing and interacting with a charm in the shell, including clicking buttons and verifying state changes.

• Test Coverage
  • Added a test that registers a charm, loads it in the shell, simulates user interaction, and checks the UI updates as expected.
  • Updated app controller to support setting identity from raw key pairs.
  • Minor update to the counter recipe to wrap the counter text in a span for easier test selection.

[bfollington]

LGTM, one question though