Skip redundant poll_max_time check when batch is already full

[chmodas]

When is_full() triggers a flush, poll_max_time() was still called unnecessarily, registering a spurious waker and potentially overwriting the flush reason from "size" to "time" in trace output.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.54%. Comparing base (ec6692b) to head (4ce5748).
⚠️ Report is 1 commits behind head on trunk.

Files with missing lines	Patch %	Lines
src/worker.rs	50.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##            trunk      #27      +/-   ##
==========================================
- Coverage   98.83%   98.54%   -0.30%     
==========================================
  Files           6        6              
  Lines         344      344              
==========================================
- Hits          340      339       -1     
- Misses          4        5       +1     

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