sql/schemachanger: constraint validation exposes internal placeholder names in error messages

**Describe the problem**

The declarative schema changer uses internal placeholder names
(`crdb_internal_constraint__name_placeholder`) while constraints
transition through intermediate states. During the `Validating` window, if
a DML statement violates the constraint, the error message surfaces this
internal name instead of the user-specified constraint name.

For example, if a user runs `ALTER DOMAIN d VALIDATE CONSTRAINT chk`, a
concurrent `INSERT` that violates `chk` would report a violation against
`crdb_internal_constraint_2_name_placeholder`.

**To Reproduce**

1. Create a table or domain with an unvalidated check constraint named `chk`.
2. Run `ALTER TABLE t VALIDATE CONSTRAINT chk` (or `ALTER DOMAIN d VALIDATE CONSTRAINT chk`).
3. While validation is in progress, insert a row that violates the constraint from another session.
4. The error references `crdb_internal_constraint_2_name_placeholder` instead of `chk`.

**Expected behavior**

Error messages should reference the user-specified constraint name (`chk`),
not the internal placeholder.

**Additional data**

The placeholder name originates in `scmutationexec` where constraints are
initially created with `tabledesc.ConstraintNamePlaceholder(id)`. The real
name is set later by a separate `SetConstraintName` operation driven by the
`ConstraintWithoutIndexName` element
(`pkg/sql/schemachanger/scplan/internal/opgen/opgen_constraint_name.go`).

This affects all constraint types in the declarative schema changer:

- **Table check constraints** — `pkg/sql/schemachanger/scexec/scmutationexec/constraint.go:75`
- **Foreign key constraints** — `pkg/sql/schemachanger/scexec/scmutationexec/constraint.go:398`
- **Unique without index constraints** — `pkg/sql/schemachanger/scexec/scmutationexec/constraint.go:530`
- **Domain check constraints** — `pkg/sql/schemachanger/scexec/scmutationexec/type.go:271`
- **Hash-sharded column constraints** — visible e.g. in `pkg/ccl/schemachangerccl/testdata/end_to_end/alter_table_alter_primary_key_rbr/alter_table_alter_primary_key_rbr.side_effects:1977`

87 end-to-end test side_effects files show this pattern.

**Possible approaches**

1. Set the real constraint name eagerly when adding the constraint, and use the `ConstraintWithoutIndexName` element only for rename operations.
2. Reorder the plan so the name-set operation runs before the constraint enters `Validating`.
3. Accept the current behavior as a brief transient window (validation is typically fast).

Epic: none

Jira issue: CRDB-67210

Epic CRDB-66030

Start with the constraint creation paths in pkg/sql/schemachanger/scexec/scmutationexec/constraint.go and type.go, then read the name operation in pkg/sql/schemachanger/scplan/internal/opgen/opgen_constraint_name.go. Reproduce concurrent validation for a named constraint and inspect the affected end_to_end side_effects files. Done means violations during validation report the user-specified name instead of the internal placeholder across the listed constraint types.

Written by the indexing model from the issue text.