Missing error handling for EventSigner methods

[nogringo]

Describe the bug

NDK assumes signing operations always succeed fast. EventSigner methods (sign,
encrypt, decrypt, encryptNip44, decryptNip44) have no error handling
throughout the codebase.

Signing can fail for multiple reasons:

• External signers (NIP-46, Amber, NIP-07): user refuses, bunker disconnected,
  timeout
• BIP340: invalid input data

When a signer fails, internal state (streams, BroadcastState, RequestState) is
not properly cleaned up.

Expected behavior

• Signer errors should be caught internally
• Streams and state properly closed on signing failure
• Exception re-thrown to caller after cleanup

Additional context

Exceptions that can be thrown:

• SignerRequestRejectedException - user explicitly rejects the request
• SignerRequestCancelledException - request cancelled by user
• Generic Exception - e.g., "Private key is required for signing"

Discovered while working on #398.