Set FEC to NONE for XPHY in platform mapping

Summary:
Agera3 will act as a retimer and does not perform FEC encoding/decoding - the IPHY (internal PHY) is responsible for FEC generation. However, the current platform mapping was incorrectly setting FEC to RS544_2N (11) for both xphyLine and xphySystem sides.

This diff hardcodes FEC to NONE (1) for XPHY in the platform mapping generation code.

**Note**: The current platform mapping CSV structure does not support different FEC values for NPU vs XPHY within the same profile. A follow-up task is filed [here](https://docs.google.com/spreadsheets/d/1TwxOU014IjbmQZLkWiF_TNy9MG8L1-wrqooPAyQSQ64/edit?gid=0#gid=0) to properly account for different FEC types in the CSV. For now, this is hardcoded in the code as a workaround.

Reviewed By: harshitgulati18

Differential Revision: D89752851

fbshipit-source-id: e17995156ec2fa8beb8545bcf51120793c2cc075

Author: Protick Bhowmick

fboss/agent/platforms/common/ladakh800bcls/Ladakh800bclsPlatformMapping.cpp

@@ -70224,14 +70224,14 @@ constexpr auto kJsonPlatformMappingStr = R"(
         "xphyLine": {
           "numLanes": 1,
           "modulation": 2,
-          "fec": 11,
+          "fec": 1,
           "medium": 3,
           "interfaceType": 0
         },
         "xphySystem": {
           "numLanes": 1,
           "modulation": 2,
-          "fec": 11,
+          "fec": 1,
           "medium": 3,
           "interfaceType": 0
         }

fboss/lib/platform_mapping_v2/generated_platform_mappings/ladakh800bcls_platform_mapping_is_multi_npu.json

@@ -70210,14 +70210,14 @@
         "xphyLine": {
           "numLanes": 1,
           "modulation": 2,
-          "fec": 11,
+          "fec": 1,
           "medium": 3,
           "interfaceType": 0
         },
         "xphySystem": {
           "numLanes": 1,
           "modulation": 2,
-          "fec": 11,
+          "fec": 1,
           "medium": 3,
           "interfaceType": 0
         }

fboss/lib/platform_mapping_v2/helpers.py

@@ -277,7 +277,8 @@ def get_platform_config_entry(
             ):
                 xphy_profile_side_config.numLanes = xphy_speed_setting.num_lanes
                 xphy_profile_side_config.modulation = xphy_speed_setting.modulation
-                xphy_profile_side_config.fec = xphy_speed_setting.fec
+                # Set FEC to NONE for XPHY (retimer PHYs like Agera3 don't do FEC encoding/decoding)
+                xphy_profile_side_config.fec = FecMode.NONE
                 if is_xphy(xphy_speed_setting.a_chip_settings.chip_type):
                     xphy_profile_side_config.interfaceType = (
                         xphy_speed_setting.a_chip_settings.chip_interface_type