fix(ci): replace breaking codesign step with signature verification

The previous ad-hoc codesign step failed with 'invalid or unsupported
format for signature' because it tried to preserve the 'linker-signed'
flag which cannot be re-signed. The native macOS build already produces
a valid ad-hoc signature via Bun's linker, so we only need to verify
the signature exists rather than re-sign it.