Fix index definition and reference roles in ALTER validation

[minleejae]

Purpose

Validate index names according to their role in an ALTER operation. ALTER TABLE child ADD INDEX ix (id) should validate a new index name and an existing column; currently metadata validation incorrectly requires ix to exist already.

Current design (As-is)

ALTER validation treats every stored index name as an existing object. This rejects new ADD names, queries an absent name for unnamed indexes, and checks a RENAME destination as existing while omitting its source.

Target design (To-be)

Extract index metadata validation into one helper:

• ADD: optional new index name, existing key columns.
• DROP INDEX/KEY: existing index name.
• RENAME INDEX/KEY: existing source name and new destination name.
• PostgreSQL PRIMARY KEY/UNIQUE USING INDEX: keep the existing reference validation path.

Quoted names are passed through unchanged. Expression-key columns continue through the expression visitor exactly once. These rules use the existing optional database-metadata validation capability; parsing behavior and dialect acceptance do not change.

Validation

• 16 new regression cases cover named/unnamed/functional/quoted indexes, ADD/DROP/RENAME, PostgreSQL USING INDEX and programmatic builders, including missing-column/source and existing-destination errors.
• Rebased onto master (166f0a27) after Separate ALTER key definitions from column drop targets #2771. The only conflict was in AlterValidator: Separate ALTER key definitions from column drop targets #2771's constraint and drop-target validation are kept, and only the index block is replaced by the role-aware helper.
• Java 17 offline Gradle spotlessApply check after the rebase: 10,110 tests, 10,085 passed, 25 skipped, zero failures/errors. Grammar, Checkstyle, PMD, SpotBugs and coverage checks pass.
• Default PMD comparison against the previous base (066f581e): no new method violations; existing ALTER validator complexity decreases (cyclomatic 14→12, NPath 1249→417).

Integration validation

Applied #2770, #2771 and all four follow-ups (#2775–#2778) together on master (066f581e), resolving overlapping validator edits while preserving their respective behavior. The integrated build passes Java 17 offline Gradle spotlessApply check: 10,141 tests, 10,116 passed, 25 skipped, zero failures/errors. The combined ALTER validator has zero findings under the default PMD complexity and field-placement rules.

Replayed 882 previously DB-validated MySQL/PostgreSQL DDL fixtures: 875 pass and the same seven existing parse gaps remain. SQL output, reparsing and AST node-type sets match the preceding integration baseline, with no new differences. This replay did not rerun the databases.

The published head passes grammar checks, Gradle and Maven on Ubuntu/Windows/macOS, and Codacy. Deployment-only jobs are skipped as expected for a PR.