fix: close all sessions in session_scope even when one close() raises (follow-up of #687)

[wdeveloper16]

Follow-up to #687. Closes #858.

Problem

In session_scope()'s finally block, if the first session.close() raises, the loop aborts and every remaining session in the cache is leaked:

for s in cache.values():
    s.close()  # raises here → remaining sessions never closed

Fix

Wrap each close() individually so all sessions are always attempted. The first exception, if any, is re-raised after the full iteration:

first_exc: Optional[Exception] = None
for s in cache.values():
    try:
        s.close()
    except Exception as exc:
        if first_exc is None:
            first_exc = exc
if first_exc is not None:
    raise first_exc

Test

Adds test_all_sessions_closed_even_when_first_close_raises to TestSessionScope: two sessions are created, the first close() is made to raise, and the test asserts both sessions had close() called and _session_cache is None.