Name the savepoint transitions in LibsqlConnection #210

Goal

Name the savepoint state transitions in LibsqlConnection. Three fields encode one state machine, and the transaction-ownership rule is buried inside list manipulation.

Depends on

#195 for the duplicate-name and outermost-release pins.

Pin

LibsqlSavepointTest covers auto-commit savepoints, implicit transactions, release of the outermost savepoint, failures, and duplicates. LibsqlConnectionTest covers the transaction paths.

Scope

• Add requireManualCommit(String action) so the rollback and release messages stay distinct.
• Add beginSavepointTransaction, discardSavepointsAfter, and closeTransactionOwnedBySavepoint.
• Delete executeTransactionStatement in favor of the shared send helper from the connection cleanup pass, or keep it if that pass has not landed.

Preserve

• A savepoint in auto-commit starts a transaction without BEGIN, and releasing the outermost savepoint commits and closes the stream.
• A failed implicit savepoint leaves auto-commit on and closes the fresh stream.
• Rollback and release in auto-commit send nothing and name the mode in the message.
• setSavepoint(null) throws, and setSavepoint() does not.
• Duplicate names resolve by lastIndexOf. Keep the list.

Acceptance

• The three state transitions are named methods, and each is called from one place.
• LibsqlConnection.java is under 740 lines.
• ./mvnw -pl libsql-jdbc -am test green with no assertion edited.
• ./mvnw -pl libsql-itests -am verify green with DOCKER_CONFIG set per AGENTS.md section 4.