feat: Support TRANSACTION_TIMEOUT system variable

[apstndb]

Implement TRANSACTION_TIMEOUT system variable for controlling transaction-level timeouts.

Reference:

• java-spanner: Supports TRANSACTION_TIMEOUT as read/write [LOCAL] variable with duration or NULL values.
• go-sql-spanner: Supports transaction_timeout as a connection property (time.Duration).

Implementation notes:

• Add TRANSACTION_TIMEOUT to system variable registry
• Apply timeout to read/write transactions
• Support [LOCAL] scoping when feat: Support SET LOCAL for transaction-scoped variables #400 is implemented

[apstndb]

Implementation guidance: apply the #739 transaction-policy snapshot rule. Snapshot the timeout at BEGIN via TransactionManager.NewTransactionOptionsBuilder; do not read it live during the transaction. Still unimplemented; tracked under #47.

[apstndb]

Reference driver data point (go-sql-spanner):

go-sql-spanner added a transaction_timeout connection property in v1.22.0 (CHANGES.md #578, same PR as statement_timeout). Exact semantics (connection_properties.go propertyTransactionTimeout):

• Type is a duration; "The timeout to apply to all read/write transactions on this connection. Setting the timeout to zero means no timeout."
• It is a distinct property from statement_timeout (per-statement) — the pair mirrors the JDBC split. spanner-mycli already has STATEMENT_TIMEOUT but no transaction-level equivalent, so TRANSACTION_TIMEOUT fills the same gap the reference driver did.
• It is a user-context property (runtime-settable) with default zero = unlimited, which matches the proposed default here.

This confirms both the name (transaction_timeout -> TRANSACTION_TIMEOUT) and the zero-means-no-timeout convention.

[apstndb]

Second reference driver data point: java-spanner's Connection API added the transaction_timeout connection property in 6.101.0 (2025-09-26; googleapis/java-spanner#4056), and 6.102.0 recalculates the remaining statement timeout after internal retries - a semantics detail worth mirroring when implementing this together with the #739 snapshot rule. With go-sql-spanner v1.22.0 also shipping transaction_timeout, both official reference drivers now support it.