Add tests to improve Rust code path coverage

[Copilot]

Generated cargo llvm-cov report and added 41 tests across 8 files targeting meaningful uncovered Rust code paths (excluding __repr__/__str__).

Coverage improvements (Rust lines)

File	Before	After
js/managers/consumers.rs	67%	95%
subscriptions/callback.rs	65%	94%
js/managers/kv.rs	76%	100%
js/message.rs	81%	99%
js/stream.rs	62%	69%
Total	82.6%	87.7%

New test files

• test_callback_subscription.py — CallbackSubscription.unsubscribe(), .unsubscribe(limit=), .drain()
• test_consumers_manager.py — ConsumersManager.delete(), .get_push(), .update() (pull/push), .pause(), .resume()
• test_js_message_acks.py — Double ack variants, nack(delay=), .next(), .progress(double=True), .term(double=True), message nullable properties
• test_stream_purge.py — Stream.purge(sequence=), Stream.purge(keep=)
• test_kv_manager.py — KVManager.update()
• test_stream_config_types.py — External, Source, Placement, ConsumerLimits constructors and properties
• test_publication_and_extras.py — Publication properties, queue group subscriptions, bytearray/memoryview payloads
• test_consumer_properties.py — PullConsumer/PushConsumer .name/.stream_name, empty fetch

Also adds *.profraw to .gitignore.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.78%. Comparing base (6e82b6f) to head (51aebed).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   82.87%   87.78%   +4.91%     
==========================================
  Files          30       30              
  Lines        3013     3013              
==========================================
+ Hits         2497     2645     +148     
+ Misses        516      368     -148     

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