Preserve user-supplied ConsumerConfig.filter_subject on subscribe

[caspervonb]

The auto-create path guarded on filter_subjects (plural) but assigned filter_subject (singular), so a config that set only the singular field had it silently overwritten with the subscribe subject.

Fixes #501.

[synadia-claude-reviewer]

Claude finished @caspervonb's task in 1m 24s —— View job

---

PR Review

• Gather PR diff and context
• Review changed files
• Post findings

---

The fix is correct and well-targeted. The push-subscribe auto-create path checked filter_subjects (plural) before assigning filter_subject (singular), so a user-supplied singular filter was silently clobbered. Adding and not config.filter_subject to the guard is the minimal, right fix.

One issue found: the same bug exists in pull_subscribe() at nats/src/nats/js/client.py:599, which this PR does not address:

# nats/src/nats/js/client.py  ~line 598-600
# Auto created consumers use the filter subject, unless filter_subjects is set.
if not config.filter_subjects:          # ← same bug: ignores user-supplied filter_subject
    config.filter_subject = subject

This should receive the identical fix: if not config.filter_subjects and not config.filter_subject:. A companion regression test for pull_subscribe would be worth adding too.

Test (nats/tests/test_js.py): the regression test is clear and sufficient for the push case. Minor note — sub is not drained or unsubscribed before nc.close(), but looking at surrounding tests that pattern is consistent with the rest of the suite so it's fine.