Cancel DefaultTransactionTimeout context on transaction completion

[00200200]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

When DefaultTransactionTimeout is set, db.Begin() creates a timeout context with context.WithTimeout, but the cancellation function was previously discarded. This left runtime timers and contexts active until the full timeout duration expired, even after the transaction finished.

This PR stores the cancellation function on the transaction statement so that:

• It is canceled when Commit() or Rollback() completes.
• It is canceled immediately if BeginTx fails with an error.
• The context timeout is preserved across clones and sessions of the transaction.
• Tests are added covering commit cancellation, rollback cancellation, and ensuring caller-supplied contexts with existing deadlines are not prematurely canceled.

Fixes #7875