[Vcode2407]

With a SERIALIZABLE connection default, getTransactionIsolation() reports SERIALIZABLE even after BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ. This change reads the isolation level from the active Spanner transaction and falls back to the connection default outside a transaction.

The mock-server regression test checks the default before BEGIN, REPEATABLE READ during the transaction, and the default again after ROLLBACK.

Fixes #14968

Validated with 218 tests across ConnectionStatementExecutorTest, JdbcConnectionTest, and TransactionMockServerTest.

[gemini-code-assist]

Code Review

This pull request updates the JDBC connection to correctly report the isolation level of the current transaction instead of always returning the default isolation level. It introduces a new getTransactionIsolationLevel() method to the Connection interface and implements it in ConnectionImpl. A test has been added to verify this behavior. The reviewer suggested preserving the check that the connection is not closed in ConnectionImpl.getTransactionIsolationLevel() to ensure that operations on a closed connection fail fast.

[gemini-code-assist]

To maintain consistency with other methods in ConnectionImpl and ensure that operations on a closed connection fail fast, we should preserve the check that the connection is not closed.

Suggested change

	public IsolationLevel getTransactionIsolationLevel() {
	return isInTransaction() ? transactionIsolationLevel : getDefaultIsolationLevel();
	}
	public IsolationLevel getTransactionIsolationLevel() {
	ConnectionPreconditions.checkState(!isClosed(), CLOSED_ERROR_MSG);
	return isInTransaction() ? transactionIsolationLevel : getDefaultIsolationLevel();
	}