feat(tidb): BATCH non-transactional DML grammar (Tier 4g)

[vsai12]

Summary

Adds parsing for TiDB's non-transactional DML statement:

BATCH [ON <col>] LIMIT <n> [DRY RUN [QUERY]] {DELETE | UPDATE | INSERT | REPLACE}

mirroring pingcap's NonTransactionalDMLStmt production ("BATCH" OptionalShardColumn "LIMIT" NUM DryRunOptions ShardableStmt).

Changes

• AST — BatchStmt node + BatchDryRunMode enum. Numeric values mirror pingcap (None=0, Query=1, SplitDML=2); locked by test.
• Lexer — BATCH/DRY/RUN keyword tokens, classified TiDBKeyword (unreserved upstream) and added to keywordCategories as kwCatUnambiguous so they remain usable as identifiers/labels/roles/lvalues.
• Parser — parseBatchStmt + dispatch case; BATCH added as a top-level completion candidate. LIMIT parses a bare NUM (not an expression), so LIMIT (1+1), LIMIT ?, and LIMIT -100 are rejected. The ON shard column must be a (qualified) column name — wildcard forms (t.*, db.t.*) are rejected, matching TiDB.
• Walker / outfuncs — BatchStmt walks ShardColumn + DML; writeBatchStmt prints all fields.
• REPLACE surfaces as *InsertStmt with IsReplace=true (omni's existing INSERT/REPLACE unification).
• Drive-by: fixed a stale for MySQL package comment in tidb/completion.

Testing

• TestBatchParse — 9 positives + 14 sibling-arm negatives (per the grammar's decorator differential, incl. wildcard shard columns)
• TestBatchAST — locks dry-run mapping, shard-column shape, REPLACE unification
• TestBatchDryRunEnumValues — locks None=0/Query=1/SplitDML=2
• TestBatchKeywordsStillIdentifiers — 9 keyword-as-identifier regression cases (no regression from the new tokens)
• TestBatchSerialize — outfuncs coverage for both DRY RUN variants + REPLACE
• TestBatchTiDBOracle — 23/23 grammar lockstep against pingcap/tidb:v8.5.5 (skips under -short)

omni has no statement-level (DML) deparse — only expression + SELECT — so parse→NodeToString→inspect is the round-trip analog used here.

Notes

• tidb/parser/{lexer,name}.go and tidb/ast/parsenodes.go are not gofmt-clean at HEAD (pre-existing). Added lines hand-match the existing alignment; these files were intentionally not gofmt -w'd to avoid ~900 lines of unrelated whitespace churn. Added lines are clean under Go 1.25 (CI) and 1.26.

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