Fix Windows Credential Manager support for Adobe Sign API keys

[cbalanoiu]

Summary

• Fix the Adobe Sign connector schema to accept secure_integration_key_key, matching the existing credential loader and documented configuration. Previously the schema required secure_integration_key, which the credential loader could not use.
• Preserve plaintext integration_key support and existing errors for conflicting credential settings or missing stored credentials.
• Add one small parameterized regression test confirming validation accepts both plaintext integration_key and secure secure_integration_key_key configuration.
• Clarify the example configuration: comment out the plaintext key when using secure storage and store the credential under the configured admin_email username.

Testing Steps

• Before the schema fix, the focused credential regression tests reproduced the validation failure for secure_integration_key_key.
• Final slim branch, on macOS with Python 3.12: python -m pytest -q --no-cov tests/test_sign_config.py tests/test_sign_engine.py tests/test_config.py: passed (41 passed, 1 skipped), including both cases of the new validation test.
• git diff --check: passed.
• Historical additional verification before slimming the diff: the same pytest command passed (49 passed, 1 skipped) with expanded tests for primary/secondary organizations, keyring lookup arguments, invalid configuration, missing stored credentials, and conflicting credentials. Those expanded tests are not in this PR's final diff; they are preserved on a separate fork branch at commit 9598ed9006c3513aa0d4e3012a5f3684d0c2176a for a possible later test-only follow-up. No follow-up PR has been submitted.
• In the historical expanded tests, keyring backend calls, Sign API client construction, and successful-case cache construction were mocked. The final small regression test only exercises schema validation. Actual Windows Credential Manager access and a live Adobe Sign sync were not tested on this macOS host.
• Suggested Windows verification: store an Adobe Sign API key as a Generic Credential named AdobeSignCreds, with the same username as admin_email. In connector-sign.yml, comment out integration_key and set secure_integration_key_key: AdobeSignCreds. Run .\user-sync.exe sign-sync -t with a build containing this change and verify credential retrieval succeeds.

Fixes #870