[Validator] Fixed string conversion in constraint violations

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10675
| License       | MIT
| Doc PR        | -
Could you some more explanations about when/why/how this is ever called? Something along the lines of:

"This method is used when a constraint error message contains the {{ value }} placeholder. By default, Symfony never includes it, but the developer can override those error messages. It's up to the developer to make sure it actually makes sense to include such string representation of submitted values in error messages."