Fix saithrift URL generation for Mellanox on internal-202411 branch sonic-net#19899

What is the motivation for this PR?
The saithrift package download fails for Mellanox platforms on internal-202411 branch because the code was hardcoding bullseye as the Debian codename for all ASIC types, but Mellanox requires the actual Debian codename from the syncd container.

How did you do it?
Modified the condition to only use hardcoded bullseye for non-Mellanox platforms on internal-202411
For Mellanox on internal-202411, the code now retrieves the actual Debian codename via docker command
This ensures the correct saithrift package URL is constructed for each platform type

How did you verify/test it?
local test

Author: XuChen-MSFT

tests/test_pretest.py

@@ -409,8 +409,8 @@ def test_update_saithrift_ptf(request, ptfhost, duthosts, enum_dut_hostname):
             pytest.fail("Unable to parse or recognize version format: {}".format(version))
 
     # Apply special codename overrides for specific internal branches
-    if branch_name == "internal-202411":
-        # internal-202411 has saithrift URL hardcoded to bullseye
+    if branch_name == "internal-202411" and asic != "mellanox":
+        # internal-202411 has saithrift URL hardcoded to bullseye for non-mellanox platform
         debian_codename = "bullseye"
     elif (branch_name.startswith("internal-") and branch_name < "internal-202405"):
         # For internal branches older than 202405, use the original URL without modification