orm: add transaction API with savepoint-backed nesting

[watzon]

Summary

This adds an ORM-level transaction API in vlib/orm. Checks off another task in #24931.

New public API:

• orm.begin(mut conn) !orm.Tx
• orm.transaction(mut conn, fn (mut tx orm.Tx) !T) !T

orm.Tx implements orm.Connection, so existing ORM surfaces work inside transactions:

• sql tx { ... }!
• orm.new_query[T](tx)

Nested transactions are implemented with savepoints rather than a second top-level transaction.

What changed

• added orm.TransactionalConnection
• added orm.Tx and orm.Savepoint
• added automatic commit/rollback helper flow via orm.transaction(...)
• added manual transaction control via orm.begin(...)
• added manual savepoint creation, rollback, and release
• added driver ORM transaction hooks for SQLite, MySQL, and PostgreSQL
• added release_savepoint(...) support in the SQLite/MySQL/PostgreSQL backends
• documented the new transaction API in the ORM README

Behavior

• root Tx.commit() commits the DB transaction
• root Tx.rollback() rolls back the DB transaction
• nested Tx.commit() releases the savepoint
• nested Tx.rollback() rolls back to the savepoint and releases it
• transaction/savepoint handles become inactive after close and return clear errors on reuse
• v1 uses each driver's default transaction mode and does not add ORM transaction syntax inside sql db {} blocks

Tests

Ran:

• ./vnew -silent test vlib/orm/
• ./vnew vlib/db/sqlite/sqlite_test.v
• ./vnew check-md vlib/orm/README.md

Also compiled:

• ./vnew vlib/db/mysql/mysql_test.v
• ./vnew vlib/db/pg/pg_test.v

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 41f89349ef

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".