[rayudu3745]

Fixes #14967.

If the database's default_transaction_isolation holds a value that this client doesn't recognize, the PostgreSQL isolation level SHOW statements failed with FAILED_PRECONDITION. They now return the value that is stored in the database, so an older client keeps working with a database that uses a newer default isolation level.

Statement (database default unknown to the client, e.g. read committed)	Before	After
SHOW TRANSACTION ISOLATION LEVEL	FAILED_PRECONDITION	read committed
SHOW transaction_isolation	FAILED_PRECONDITION	read committed
SHOW default_transaction_isolation	FAILED_PRECONDITION	read committed

Changes

• DatabaseMetadata stores the default_transaction_isolation value as returned by Spanner, and derives the isolation level from it.
• New @InternalApi method DatabaseClient#getDatabaseDefaultIsolationLevelValue() returns that value.
• The SHOW statements return it when the isolation level is UNRECOGNIZED.

[gemini-code-assist]

Code Review

This pull request introduces a new method getDatabaseDefaultIsolationLevelValue() to DatabaseClient and its implementations to retrieve the raw string value of the default transaction isolation level. This allows handling unrecognized isolation levels (such as "read committed" in PostgreSQL) without throwing an exception. The feedback recommends adding @Nullable annotations to the overridden implementations of getDatabaseDefaultIsolationLevelValue() in DatabaseClientImpl and MultiplexedSessionDatabaseClient to match the interface definition. Additionally, it suggests wrapping the call to getDatabaseDefaultIsolationLevelValue() in ConnectionStatementExecutorImpl with Objects.requireNonNull to prevent potential null-safety warnings or runtime exceptions since the enclosing method returns a non-nullable string.