Improve executor pool configuration and fix idle GIL contention

[benoitc]

Summary

• Add MIN_EXECUTORS (2) and increase MAX_EXECUTORS to 32 for more flexibility
• Calculate default executors dynamically as (dirty_cpu_schedulers div 2) + 1
• Fix GIL contention by acquiring GIL only when processing actual work, not while idle

Root Cause

Idle executor threads were continuously cycling through GIL acquire/release, competing with dirty schedulers running actual Python work via the context-based API (py:call()).

Fix

Changed multi_executor_thread_main() to:

1. Not acquire GIL at thread start
2. Wait for work using only pthread mutex/cond (no GIL)
3. Acquire GIL only when there's actual work to process
4. Release GIL before signaling completion