test: serde + transaction integration suites

[MattJackson]

Integration test coverage for the new feature surface.

• serde round-trip integration suite.
• Transaction integration suite (commit / rollback / isolation).
• Registers the serde and command test targets.

Squashed into a single commit (submitter's own work). Completes the sync — after this lands, tiberius-rs/tiberius's code matches the tiberius-ng feature set exactly (identity/branding intentionally unchanged).

Supersedes: none.

Sequential series — merge last, after #432–#436. Based on main; diff reduces to its own delta as the earlier PRs land.
Reviewer note: rebase-merge or merge-commit, not squash.

[MattJackson]

Superseded by #444. Re-opened as a native GitHub stacked PR (stack #445) per @aqrln's request in #440 — the fork origin of these PRs made a native stack impossible (GitHub can't add fork-head PRs to a stack), so the branches now live in tiberius-rs/tiberius directly. Same commits, same authorship. Please review #444 instead.