fix: keep smtp_pass consistent in auth config.

[JEETDESAI25]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

Closes #202

When using smtp_pass in auth config, Terraform fails with "Provider produced inconsistent result after apply" because readAuthConfig() marshals the API response (AuthConfigResponse) with different JSON tags than the update schema (UpdateAuthConfigBody), causing state mismatch.

What is the new behavior?

• Both readAuthConfig() and updateAuthConfig() now convert API responses to UpdateAuthConfigBody before persisting state, ensuring consistent JSON marshaling.
• Added convertAuthResponse() and copySensitiveFields() helpers to handle type conversion and preserve write-only fields. Includes regression test with request validation.

Additional context

All tests pass (7/7). Verified with golangci-lint (0 issues).