[KAPUIST]

When a subclass adds a different validator to an inherited property, the parent's validators are incorrectly discarded because they share the customValidation metadata type. For example, adding @IsNotEmpty() in a subclass can silently remove the parent's @MinLength() check.

Compare custom validation metadata by validator name, falling back to the constraint class for unnamed @Validate() metadata. This preserves distinct inherited validators while retaining child overrides of the same validator and the existing handling of other metadata types.

Fixes #2609.

Validation:

• Added five regression tests covering distinct validators across inheritance levels, simultaneous parent/child errors, same-validator overrides, custom constraint classes, and validation groups.
• Three regression tests failed before the fix; all 811 tests across 15 suites pass after the fix.
• CommonJS build, source lint, changed-file formatting, and git diff --check pass.
• A separate runtime check with NestJS PartialType confirms inherited constraints are retained and optional fields remain optional.

This change addresses inherited constraint loss; it does not change PartialType optionality or deduplication between multiple ancestor classes.

[KAPUIST]

Context on prior work: #748 and #2641 address the same underlying inheritance/deduplication bug and are still unmerged. #2641 is essentially an updated/rebased version of #748.

This PR takes a narrower approach: it changes deduplication only for customValidation metadata in MetadataStorage, using validator name (with constraintCls as a fallback for unnamed @Validate() constraints), while leaving the handling of other metadata types unchanged. I opened it separately because #2609 is still reproducible on current develop, and this smaller change adds focused regression coverage for the current behavior.

If maintainers would prefer this work to be folded into #2641 instead, I’m happy to adjust.