DELCUS follow-ups: commarea custno cross-validation, CommDelFailCd placeholder

[a2chang]

Follow-ups from PR #21 (DELCUS) review at the 3-round cap.

1. (medium) Validate CommCustno body matches path on DELETE

• File: src/main/java/com/augment/cbsa/web/delcus/DelcusController.java:53
• customerNumber is parsed from the URL path; the body commarea's CommCustno is currently ignored. A mismatched request (path vs body) silently deletes the path target. When delCus.commCustno is present, validate it equals the parsed path value or return 400.

2. (low) Use a single-space placeholder for CommDelFailCd on success

• File: src/main/java/com/augment/cbsa/web/delcus/DelcusController.java:102
• CommDelFailCd is a fixed-width 1-char commarea value. The success response currently returns the empty string. Return a single space to preserve length-1 contract for clients porting from COBOL.

Both are non-blocking; merged at cap.