Handler-level sign_event → verify-signature instrumented test

[kwsantiago]

Gap

The existing FrostSigningIntegrationTest provisions a real share and round-trips get_public_key, but there is no test that drives an actual sign_event end-to-end and verifies the resulting Schnorr signature. So a regression that breaks signing (while leaving key export intact) would not be caught.

Proposal

Add a wisp-style instrumented test (real state, real protocol, no UI) that:

1. Provisions a share via the no-auth path (AndroidKeystoreStorage(context, requireUserAuth = false) → frostGenerate → importShare), as FrostSigningIntegrationTest already does.
2. Drives Nip55Handler with a sign_event request.
3. Asserts a valid Schnorr signature over the event (verify against the group pubkey).

Notes

• Keys are 2-of-2 FROST, so a real signature needs both shares coordinated within the test.
• This is deliberately handler-level rather than a UI round-trip through Nip55Activity: the Activity path requires keystore user-auth that the CI emulator (google_apis, no enrolled fingerprint) cannot satisfy, which would make a UI sign test flaky.
• Complements the Compose flow tests added in Add Compose UI flow tests for approval, unlock, and launch #352.