[Quality] Document the public CDC environment utility contract

[seonghobae]

Quality gap

Protected develop@622e5e6c3d534f230c390f10e3832efadfc01825 exposes public cdc-service utility EnvUtils and its public getEnv(String, String) / requireEnv(String) methods without beginner-readable Javadocs. The methods participate directly in deployment configuration and fail-closed required-variable handling, so their default/blank/missing semantics should be discoverable without reading implementation or tests.

Repository policy requires beginner-readable public documentation. This is path-disjoint from active CDC behavior, registry, replica, mapper, bootstrap, and connector work.

Test-first acceptance

1. Add a focused source contract before production source changes, preserving the existing public static signatures and current missing/blank semantics while requiring class/method Javadocs.
2. Observe hosted RED only at the missing documentation boundary after normal production/test compilation.
3. Add only class and public-method Javadocs; do not change environment lookup, blank handling, returned values, exception type, credentials, or configuration names.
4. Rerun focused/full applicable verification.
5. Keep the PR Draft until shared source-identity, scanner-completeness, non-vacuous coverage, and independent-review gates are satisfied.

Non-goals

No new normalization, secret masking, config aliases, exception rewrite, dependency change, or cross-repository mutation.