fix(spanner): fail transactionIdFuture when inline-begin R/W statement fails to start (P0-4)

[fornwall]

Issue (review P0-4)

A read-write TransactionRunnerImpl.TransactionContextImpl overrides onError/onDone but not onStartFailed (the base AbstractReadContext.onStartFailed is a no-op).

When the begin-carrying first statement of an inline-begin read-write transaction fails to even start its streaming RPC — e.g. the client was closed, the executor rejected the task, or an interceptor threw — AbstractReadContext#startStream reports it through onStartFailed(withBeginTransaction, t), not through onError/onDone:

try {
  call = rpc.executeQuery(/* … */);      // throws synchronously
} catch (RuntimeException | Error t) {
  onStartFailed(withBeginTransaction, t); // no-op for R/W transactions
  throw t;
}

Because onStartFailed is a no-op for the R/W path, the transactionIdFuture that getTransactionSelector() created for the inline begin never completes. The synchronous throw propagates out of the user's TransactionCallable, and runInternal then calls txn.rollback(). rollbackAsync() waits on the never-completing transactionIdFuture, and rollback()'s unbounded get() blocks the application thread forever. commit()'s waiter has the same exposure.

The read-only MultiUseReadOnlyTransaction already handles this exact case by overriding onStartFailed to fail its transactionIdFuture.

Fix

Override onStartFailed in TransactionContextImpl to complete transactionIdFuture exceptionally when withBeginTransaction is set, mirroring the existing onError/onDone handling. Waiters (and rollback()/commit()) are then released. Since no transaction was ever begun on the backend, rollbackAsync() short-circuits to an empty result via its existing catching(...) and issues no Rollback RPC.

Test

Adds TransactionContextImplTest#testRollbackDoesNotHangWhenInlineBeginStatementFailsToStart, which reproduces the production sequence (getTransactionSelector() → onStartFailed(true, t)) on an inline-begin context and asserts:

• transactionIdFuture is completed exceptionally with the start failure, and
• rollbackAsync() returns promptly (empty result, no Rollback RPC sent).

Without the fix the test fails deterministically at the isDone() assertion (and rollback() would otherwise hang). Verified failing on unmodified code and passing with the fix.

TransactionContextImplTest, TransactionRunnerImplTest, and InlineBeginTransactionTest (109 tests) all pass; both changed files pass google-java-format.

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