polish(validation): replace requireNonNull with bean validation (#10, #15)

[a2chang]

Closes #10, closes #15.

Why

Two related polish items:

• CREACC DTOs: replace Objects.requireNonNull in record constructors with bean-validation so missing fields return 400, not 500 #15 — Objects.requireNonNull(...) calls inside the canonical
  constructor of a request-DTO record are executed by Jackson during
  deserialization. When a required field is missing from the JSON body,
  Jackson sees a null component, the constructor throws
  NullPointerException, and CbsaExceptionHandler#handleUnexpected
  surfaces it as a 500 with abend code UNEX instead of the intended
  400 Validation failed. Bean-validation annotations on the
  components, combined with @Valid on the controller parameter and
  every nested DTO/Key reference, give us the proper 400.

• CRECUST: tighten CommName validation (no blank/whitespace via empty title) #10 — VALID_TITLES in CrecustService contained an empty
  string, which let an empty/whitespace-only CommName silently bypass
  title validation. Tighten by (a) requiring @NotBlank at the wire
  boundary on CrecustCommareaRequestDto.commName and on
  UpdcustCommareaRequestDto.commName/commAddress, and (b) removing
  the empty-string entry from VALID_TITLES so the service-layer guard
  also rejects blank names with fail code T.

What

• Drop Objects.requireNonNull from the canonical constructor of every
  request DTO and nested Commarea/Key DTO that participates in JSON
  deserialization: CrecustRequestDto, CrecustCommareaRequestDto,
  CrecustKeyDto, CreaccRequestDto, CreaccCommareaRequestDto,
  CreaccKeyDto, DbcrfunRequestDto, UpdcustRequestDto,
  UpdcustCommareaRequestDto, DelcusRequestDto. Existing @NotNull
  / @Valid annotations carry the contract.
• Add @NotBlank on CrecustCommareaRequestDto.commName and on
  UpdcustCommareaRequestDto.commName/commAddress.
• Remove the empty-string entry from CrecustService.VALID_TITLES so
  the service-layer fallback also rejects blank names.
• Document the rule in docs/translation-rules.md §6 so future program
  translations follow the same pattern.

Tests

• CrecustControllerWebMvcTest:
  • missingRequiredFieldReturnsBadRequest — omitting CommName
    yields 400 (previously would have been 500/UNEX).
  • blankCommNameReturnsBadRequest — whitespace-only CommName
    yields 400 via @NotBlank.
• CrecustServiceUnitTest:
  • rejectsBlankCustomerNamesAtTitleValidation — locks in the
    service-layer rejection of blank names with fail code T.

./mvnw verify: 205/205 green locally.

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^{Test Coverage Guardian 🧪}

✅ Test coverage looks good. The new/changed behavior in this PR has adequate test coverage. No additional tests needed.

The PR refactors validation from Objects.requireNonNull to bean validation annotations and includes three focused tests:

• missingRequiredFieldReturnsBadRequest — validates 400 response for missing required fields (issue CREACC DTOs: replace Objects.requireNonNull in record constructors with bean-validation so missing fields return 400, not 500 #15)
• blankCommNameReturnsBadRequest — validates 400 response for whitespace-only names (issue CRECUST: tighten CommName validation (no blank/whitespace via empty title) #10)
• rejectsBlankCustomerNamesAtTitleValidation — validates service-layer rejection of blank names (issue CRECUST: tighten CommName validation (no blank/whitespace via empty title) #10)

All critical validation paths are exercised.

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