docs: annotate QueryModifiers and SchemaAndTable as @Immutable

[velo]

Annotation only, no behaviour change.

@Immutable is RetentionPolicy.CLASS and marks the types in this codebase
that are already value-shaped: final fields only, value-based equals, no
identity dependence. PathMetadata, JoinExpression, JoinFlag,
OrderSpecifier, Template and its Element subclasses, ConstantImpl and
friends all carry it.

Two types that meet the same bar were missing it:

• QueryModifiers — final class, two final fields, value equals/hashCode
• SchemaAndTable — two final fields, value equals/hashCode

This keeps the annotated set an accurate inventory of the value-shaped types,
which is the list you would work from for any future value-class migration
(JEP 401), and it is useful documentation regardless.

Deliberately not included, since both are breaking changes and belong in their
own discussion:

• marking these final — nothing in the repo extends either, but downstream
  code may
• converting them to records — changes accessor names (getSchema() →
  schema()) and the serialized form

Testing

./mvnw -Pdev -pl :querydsl-sql -am test: 3952 tests, 0 failures.

🤖 Generated with Claude Code

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