extmod/zephyr_kernel: Restore k_yield() in mutex unlock for fair scheduling.

Restores k_yield() call after k_sem_give() in mp_thread_mutex_unlock() to
match ports/zephyr pattern (mpthreadport.c:272). Without k_yield(), when
GIL is released, current thread continues execution and can immediately
re-acquire GIL before waiting threads get scheduling opportunity, violating
fair GIL access semantics.

With same thread priority (previous commit), k_yield() ensures waiting
threads get immediate opportunity to acquire GIL after release, implementing
proper fair scheduling behavior.

Test results: 27/34 passing (unchanged from priority fix alone).
Remaining 7 failures have different root causes.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

Author: pi-anl

extmod/zephyr_kernel/kernel/mpthread_zephyr.c

@@ -592,10 +592,7 @@ int mp_thread_mutex_lock(mp_thread_mutex_t *mutex, int wait) {
 // Unlock mutex
 void mp_thread_mutex_unlock(mp_thread_mutex_t *mutex) {
     k_sem_give(&mutex->handle);
-    // NOTE: k_yield() removed - was causing deadlock during thread creation
-    // When main thread creates new thread and releases thread_mutex, yielding
-    // immediately causes new thread to run and block on GIL (held by main),
-    // creating deadlock. Rely on timeslicing for context switches instead.
+    k_yield();  // Yield to allow waiting threads fair access (matches ports/zephyr)
 }
 
 // Recursive mutex functions (only compiled when GIL is disabled)