fix(drizzle-kit): use DROP INDEX IF EXISTS for SQLite/Turso index drops

[muhammadosama984]

Summary

• make LibSQLModifyColumn emit DROP INDEX IF EXISTS when dropping indexes during Turso/libSQL alter-column/check-constraint flows
• make SqliteDropIndexConvertor emit DROP INDEX IF EXISTS for explicit SQLite/Turso drop_index statements
• update affected SQLite/libSQL push + statement tests and add regression coverage for multi-table alter-column flows to assert idempotent index-drop SQL

Root Cause

LibSQLModifyColumn.convert() can run multiple times in a single drizzle-kit push execution. It currently emits bare DROP INDEX statements; repeated drops on the same index then fail with no such index in SQLite/libSQL.

SqliteDropIndexConvertor.convert() also emits bare DROP INDEX, which is non-idempotent.

Fix

Use DROP INDEX IF EXISTS in both paths so repeated drops become no-ops.

Validation

• pnpm --filter drizzle-kit exec vitest tests/libsql-statements.test.ts tests/push/libsql.test.ts tests/push/sqlite.test.ts --run

Closes #5564