refactor: migrate services and middleware away from ValidateHelper

[maia-andre]

Resolves: #8355

📝 Summary

Replaces the ValidateHelper compatibility façade with the focused validators in the four remaining central consumers, following the mapping defined in the issue:

consumer	validators injected	calls migrated
RequestSignatureService	FileInputValidator, SigningRequestValidator, SignerValidator	validateNewFile, validateFileStatus, validateIdentifySigners, normalizeRequestSigners (×3, same SignerValidator instance)
AccountService	IdentityDocumentValidator, FileInputValidator	userCanApproveValidationDocuments (×2), validateBase64 (×2), constant TYPE_VISIBLE_ELEMENT_USER now read from FileInputValidator
SignFileService	IdentityDocumentValidator, SigningRequestValidator, SignerValidator	userCanApproveValidationDocuments, fileCanBeSigned, validateUuidFormat, validateSigner, validateRenewSigner
InjectionMiddleware	SigningRequestValidator, SignerValidator	canRequestSign, validateSigner, validateSignerUuid

Dependency migration only:

• no validation rule moves between validators;
• no new façade, wrapper or interface;
• SignFileService::validateSigner() and validateRenewSigner() keep their signatures and only change the delegate;
• InjectionMiddleware attributes, authorization flow, exception handling and redirects are untouched;
• the new constructor parameters take the position the ValidateHelper parameter had, so the positional constructor calls in the tests stay readable.

After this change the only ValidateHelper consumers left in lib/ are the three controllers covered by #8353, plus the façade itself. In tests/, only PageControllerTest (for PageController, also #8353) and ValidateHelperTest still reference it.

🧪 How to test

Ran in the devcontainer (PHP 8.3):

check	result
php -l on the 8 files	clean
php-cs-fixer --dry-run on the 8 files	nothing to fix
psalm on the 4 production files, before vs. after	same 8 pre-existing MissingDependency errors (local psr/* stubs), no new issue
RequestSignatureServiceTest + AccountServiceTest + SignFileServiceTest + InjectionMiddlewareTest	OK, 214 tests, 554 assertions
ValidateHelperTest + PageControllerTest + Validation/*	OK

Search to confirm the remaining references:

grep -rln ValidateHelper lib tests

⚙️ API / Back‑end changes

• Constructor dependencies of the four classes changed from ValidateHelper to the focused validators (container-resolved; no public API change)
• Unit tests updated: the ValidateHelper mock is replaced by one mock per focused validator, every existing expectation moved to the validator that owns the method, no assertion removed or weakened

🚧 Tasks

• Decide the merge order with test: improve unit tests coverage for AccountService (Infection) #8142, which adds five expectations on $this->validateHelper in AccountServiceTest (userCanApproveValidationDocuments, validateBase64 with ValidateHelper::TYPE_VISIBLE_ELEMENT_USER). Whichever merges second needs a small rebase; I can do it on either side.

✅ Checklist

• I have read and followed the contribution guide.
• Commit signed off (DCO).
• No new façade, wrapper or interface; no validation rule moved; no test removed or weakened.

🤖 AI (if applicable)

• The content of this PR was partially or fully generated using AI