[IT9PE1-30741] Add DutchKvkNumber constraint, validator and provider

[fguimier]

What

Add the DutchKvkNumber constraint — the 8-digit Dutch KVK number (Kamer van Koophandel nummer) — following the existing BelgianEnterpriseNumber pattern:

• DutchKvkNumber constraint + DutchKvkNumberValidator: format-only (^\d{8}$), single WRONG_FORMAT_ERROR. KVK numbers carry no public check digit, so no checksum branch.
• DutchKvkNumberProvider: maps the dutchKvkNumber DBAL type to the constraint (autoconfigured via FieldConstraintsSetProviderInterface).
• composer.json: assoconnect/doctrine-types-bundle ^v2.16 → ^2.18 (the provider imports DutchKvkNumberType::NAME, added in v2.18.0).
• README: new validator entry.

Refs IT9PE1-30741

Test plan

• DutchKvkNumberValidatorTest (shared ConstraintValidatorTestCase): valid null/''/34179503 (Adyen docs example) + leading-zero cases; 7 invalid format cases each asserting WRONG_FORMAT_ERROR.
• DutchKvkNumberProviderTest (shared FieldConstraintsSetProviderTestCase): dutchKvkNumber type → [new DutchKvkNumber()].
• vendor/bin/phpcs, vendor/bin/phpstan, vendor/bin/phpunit (318 tests) green in the open-source container.

Note: CI resolves doctrine-types-bundle from Packagist — v2.18.0 was tagged today; if the highest jobs fail on the ^2.18 constraint, Packagist has not indexed the release yet (trigger an update on the package page and re-run).

Shipping

Merging this PR ships nothing: it needs a semver tag / GitHub release (v2.49.0, after the ItalianFiscalCode PR lands too), then a
composer require assoconnect/validator-bundle:^2.49 bump PR in the backend. Two reviews total.

[fguimier]

🤖 Review gate — 0 error / 0 warning / 2 info

Human review needed: no — faithful pattern copy of BelgianEnterpriseNumber with full tests and a verified dependency floor bump; no migrations/money/security/hotspot involvement; can be merged without a peer review.

📋 Card audit: 7 done

⚠️ Self-review by the author is mandatory before merging, regardless of this verdict.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud