perf: refactor executePlan to try to avoid constantly entering Tokio runtime

[mbutrovich]

Which issue does this PR close?

N/A.

Rationale for this change

Performance profiling reveals significant execution time spent in tokio::runtime::Runtime::enter() overhead. The polling loop in executePlan was entering and exiting the Tokio runtime on every iteration, causing significant performance degradation.

What changes are included in this PR?

Refactored the polling loop in Java_org_apache_comet_Native_executePlan to enter the Tokio runtime once for the entire loop instead of on each iteration:

• Moved get_runtime().block_on() to wrap the entire loop instead of calling it inside the loop
• Used tokio::task::block_in_place() around JNI calls in the Poll::Pending branch to ensure thread safety (JNI env is thread-local and cannot safely migrate between Tokio worker threads)
• Added TODO comment documenting potential future optimization if JNI thread safety can be guaranteed without block_in_place

How are these changes tested?

Existing tests.

[parthchandra]

lgtm. pending ci.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.61%. Comparing base (f09f8af) to head (8b83a08).
⚠️ Report is 777 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2938      +/-   ##
============================================
+ Coverage     56.12%   59.61%   +3.48%     
- Complexity      976     1379     +403     
============================================
  Files           119      167      +48     
  Lines         11743    15430    +3687     
  Branches       2251     2550     +299     
============================================
+ Hits           6591     9198    +2607     
- Misses         4012     4946     +934     
- Partials       1140     1286     +146     

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