Expand test coverage and fix incorrect stub type annotations

[Copilot]

Test coverage had gaps across several API surfaces, and a handful of stub type annotations were wrong in ways that would mislead type checkers and users.

Stub fixes

• kv.pyi: KVConfig.description was annotated str (should be str | None); max_age and limit_markers were float | None (Rust Duration exposes as Python timedelta, not float)
• stream.pyi: SubjectTransform.__new__ removed — the class has no Python constructor (Rust-internal only); StreamInfo.created corrected from float to int (Rust i64 Unix timestamp)

New tests (+33, 268 → 301)

Nats client properties — addr, token, nkey, user_and_pass, custom_inbox_prefix, read_buffer_capacity, sender_capacity, max_reconnects: default values and custom construction both covered.

Subscription context manager — SubscriptionCtxManager.detatch() was untested.

JetStream — js.counters manager property.

Stream — stream.name, StreamInfo.created, StreamState.subjects_count, StreamState.first/last_timestamp, StreamState.consumer_count.

Consumers — PullConsumer.consume() and PushConsumer.consume() context managers were never exercised.

JetStreamMessage properties — __len__, length, published, pending, delivered, consumer_sequence, stream_sequence, consumer, stream.

KV — KVEntry.seen_current, KVConfig.max_age timedelta round-trip, KVConfig.description None.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.00%. Comparing base (1e67c68) to head (27b8c6b).
⚠️ Report is 1 commits behind head on feature/otlp-js-consumers.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feature/otlp-js-consumers      #49      +/-   ##
=============================================================
+ Coverage                      85.41%   87.00%   +1.58%     
=============================================================
  Files                             32       32              
  Lines                           3100     3100              
=============================================================
+ Hits                            2648     2697      +49     
+ Misses                           452      403      -49     

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