Add test coverage and docstrings for counters module

[Copilot]

The counters module had zero test coverage and missing docstrings on CounterEntry, Counters, and all Counters methods.

Tests (python/tests/test_counters.py)

• CountersManager CRUD: create, create_or_update, get, delete, update
• Counter operations: add, incr, decr, get including negative values, multiple increments, mixed incr/decr, separate subjects
• Edge cases: nonexistent key raises, config defaults, entry attribute types

Docstrings (counters.pyi)

• Added class docstrings for CounterEntry and Counters
• Added method docstrings for add, incr, decr, get with param/return/raises

Stub fix (managers.pyi)

• CountersManager.update() parameter type corrected from CountersConfig → StreamConfig to match the Rust implementation, which accepts StreamConfig:

// src/js/managers/counters.rs:87
pub fn update<'py>(&self, py: Python<'py>, config: StreamConfig) -> ...

Note: callers must set allow_direct=True and allow_message_counter=True on the StreamConfig to avoid NATS rejecting the update.