fix: use signer's own Free() instead of system libc free on Windows

On Windows, ctypes.util.find_library('c') returns msvcrt.dll, but the Go
signer DLL (cross-compiled with MinGW) may use a different C runtime or
statically link its own CRT. Calling msvcrt's free() on memory allocated
by a different CRT's malloc() is undefined behavior (crashes, heap
corruption). Fix by using the signer's own exported Free() function which
is guaranteed to use the matching allocator.

Requires the corresponding lighter-go change that exports the Free function.

Co-Authored-By: Mihail  <mlavric64@gmail.com>

Author: devin-ai-integration[bot]

lighter/signer_client.py

@@ -20,7 +20,6 @@
 from lighter.models.resp_send_tx import RespSendTx
 from lighter.models.resp_send_tx_batch import RespSendTxBatch
 from lighter.transactions import CreateOrder, CancelOrder, Withdraw, CreateGroupedOrders
-from lighter.libc import free
 
 CODE_OK = 200
 
@@ -99,8 +98,10 @@ def decode_and_free(ptr: Any) -> Optional[str]:
             return c_str.decode('utf-8')
         return None
     finally:
-        # Free the memory allocated by the C library
-        free(ptr)
+        # Free the memory using the signer's own Free function to ensure
+        # the same C runtime that allocated the memory also frees it.
+        # This is critical on Windows where different CRTs have separate heaps.
+        __signer.Free(ptr)
 
 
 def __populate_shared_library_functions(signer):
@@ -174,6 +175,9 @@ def __populate_shared_library_functions(signer):
     signer.SignApproveIntegrator.argtypes = [ctypes.c_longlong, ctypes.c_uint32, ctypes.c_uint32, ctypes.c_uint32, ctypes.c_uint32, ctypes.c_longlong, ctypes.c_longlong, ctypes.c_int, ctypes.c_longlong]
     signer.SignApproveIntegrator.restype = SignedTxResponse
 
+    signer.Free.argtypes = [ctypes.c_void_p]
+    signer.Free.restype = None
+
 
 def get_signer():
     # check if singleton exists already