PYTHON-6035

Second of four PRs splitting #2964. Base is PYTHON-5947-otel-1-operations.

Changes in this PR

Wraps a transaction's operation spans in a "transaction" span, per the OpenTelemetry driver specification, so the operations belonging to one transaction group under it instead of appearing as unrelated siblings.

• A "transaction" span covers start_transaction() through commit_transaction()/abort_transaction(), with the operations inside nested under it.
• A retried with_transaction() produces one span for the whole call rather than one per attempt.
• A nested or concurrent with_transaction() call on the same session now raises InvalidOperation instead of leaking the outer call's span.
• Committing or aborting a transaction that never started on the server closes its span instead of leaving it open.
• A transaction span parents only the operations of its own session, so a concurrent unrelated session cannot be captured by it.

with_transaction's retry loop moves into a helper to keep the span handling readable; diffing with whitespace ignored shows the loop body untouched.

Test Plan

• 10 new tests each side: operations nesting under the transaction span, one span across with_transaction retries, retried commits, early-return commit and abort paths, spans across two sessions, and the nested-call guard.
• Locally against a replica set: 20 new tests pass, along with test_transactions.py and PR 1's 98 otel tests. just lint clean.

Transaction spans only run on a replica set, which PR 1's expanded Evergreen variant covers in full.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?