fix: promote certificate validity dates to signer root

[rodrigodev]

Fixes #7825

Problem

On the document validation panel, every signer shows "No expiration date" even though the signer's leaf certificate carries a valid notAfter.

Root cause

The Apr-2026 refactor that extracted CertificateSignersMergeService::processChainData() stopped promoting the end-entity (leaf) certificate's valid_from/valid_to to the top-level signer object. Before that refactor, SignersLoader merged chain[0]'s fields into the signer root via array_merge.

The validation UI (src/components/validation/SignerDetails.vue) and the SignerDetail OpenAPI schema both read valid_from/valid_to from the signer root, so the panel always rendered "No expiration date".

A unit test (testLoadSignersFromCertDataPreventsDuplicateFormattedDates) had accidentally codified the buggy behavior by asserting root-level valid_to should be absent — contradicting the existing contract from Oct-2025 (FileServiceTest + OpenAPI).

Fix

• CertificateSignersMergeService::processChainData(): promote the ISO UTC valid_from/valid_to from chain[0] to the signer root. The chain still carries the full per-certificate dates for the certificate-chain view.
• Tests:
  • New regression test testMergePromotesLeafCertValidityDatesToSignerRoot.
  • Updated testLoadSignersFromCertDataPreventsDuplicateFormattedDates to assert the corrected contract (ISO dates present at root, locale-formatted backend strings and technical fields still excluded).
  • Behat: tests/integration/features/file/validate.feature now asserts valid_from/valid_to on signers[0] of the validate endpoint response.

Validation

• Full PHPUnit suite: 3518 tests / 8564 assertions OK (run before any behat/environment activity)
• Focused suites for the changed area pass (31 tests / 117 assertions)
• php -l, php-cs-fixer dry-run, psalm (no errors)
• Behat feature not executed locally (dev-container PHP built-in server + memcache limitation documented in AGENTS.md); the added jq assertion matches the established test() pattern used in page/index.feature