fix(appkit): use retry client for Lakebase V1 transaction retry

[fallintoplace]

Summary

• use the retry PostgreSQL client for retry transaction BEGIN and COMMIT after credential rotation
• guard the original transaction client release so retry paths do not release it twice
• add a regression test covering auth-failure transaction retry behavior

Root Cause

The auth retry path opened a retry client and passed it to the transaction callback, but still ran transaction boundaries on the original client after releasing it. That could put callback work and transaction control on different clients, or fail because the original client was already released.

Validation

• pnpm exec vitest run packages/appkit/src/connectors/tests/lakebase-v1.test.ts
• pnpm exec biome check packages/appkit/src/connectors/lakebase-v1/client.ts packages/appkit/src/connectors/tests/lakebase-v1.test.ts
• pnpm --filter @databricks/appkit typecheck
• git diff --check

Closes #392