fix: use run_in_executor in LLMAgent.astep() to avoid blocking the ev…

[khansalman12]

Summary

astep() in LLMAgent was calling self.step() synchronously when a subclass only defines step(), blocking the event loop during parallel stepping.

Bug / Issue

Fixes #140

When astep() falls back to self.step() directly, any blocking LLM API call inside step() freezes the entire asyncio event loop. Other agents cannot run until the current one completes, making parallel execution no faster than sequential.

Implementation

In llm_agent.py, replaced the direct blocking call:

# Before
self.step()

# After
loop = asyncio.get_running_loop()
await loop.run_in_executor(None, self.step)

run_in_executor offloads the sync call to a thread pool, keeping the event loop free.  
Using asyncio.get_running_loop() (not the deprecated get_event_loop()) since `astep()` is always called from within an async context.

Testing
-------
Added 3 regression tests in `test_llm_agent.py`:

1. `test_astep_fallback_runs_sync_step_without_blocking`
   - Verifies `step()` is called via executor fallback.

2. `test_astep_fallback_does_not_block_concurrent_agents`
   - Verifies both agents complete with `asyncio.gather`.

3. `test_astep_with_native_astep_not_affected`
   - Verifies agents with their own `astep()` are unaffected.

All 3 tests pass.

Additional Notes
----------------
None.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.89%. Comparing base (a8479d3) to head (63b305c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   89.26%   89.89%   +0.62%     
==========================================
  Files          19       19              
  Lines        1472     1474       +2     
==========================================
+ Hits         1314     1325      +11     
+ Misses        158      149       -9     

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