[YANG] Update device_metadata to add suppress-fib-pending field (#19329)

Why I did it
BGP pending FIB suppression is introduced in #12853, hence updating the YANG model.

Work item tracking
Microsoft ADO (number only): 28398135
How I did it
Added a new field 'suppress-fib-pending' in the DEVICE_METADATA YANG model table.
Added UT for YANG model changes.

How to verify it
Run PR tests.
Apply empty.json file to verify:

admin@str2-7050cx3-acs-02:~$ sudo config apply-patch empty.json 
Patch Applier: Patch application starting.
Patch Applier: Patch: []
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating all JsonPatch operations are permitted on the specified fields
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
Patch Applier: Sorting patch updates.
Patch Applier: The patch was converted into 0 changes.
Patch Applier: Applying 0 changes in order.
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Patch Applier: Patch application completed.
Patch applied successfully.
admin@str2-7050cx3-acs-02:~$ 
Signed-off-by: zitingguo-ms zitingguo@microsoft.com

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -364,7 +364,9 @@
                 "bgp_adv_lo_prefix_as_128": "true",
                 "yang_config_validation": "disable",
                 "rack_mgmt_map": "dummy_value",
-                "timezone": "Europe/Kiev"
+                "timezone": "Europe/Kiev",
+                "synchronous_mode": "enable",
+                "suppress-fib-pending": "disabled"
             }
         },
         "VLAN": {

src/sonic-yang-models/tests/yang_model_tests/tests/device_metadata.json

@@ -146,5 +146,15 @@
     "DEVICE_METADATA_INVALID_CREATE_ONLY_CONFIG_DB_BUFFERS": {
         "desc": "Verifying invalid create_only_config_db_buffers value",
         "eStrKey": "InvalidValue"
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_ENABLED": {
+        "desc": "Enable bgp-suppress-fib-pending"
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_DISABLED": {
+        "desc": "Disable bgp-suppress-fib-pending"
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_ENABLED_SYNCHRONOUS_MODE_DISABLED": {
+        "desc": "Enable bgp-suppress-fib-pending when synchronous mode is disabled",
+        "eStr": ["ASIC synchronous mode must be enabled in order to enable suppress FIB pending feature"]
     }
 }

src/sonic-yang-models/tests/yang_model_tests/tests_config/device_metadata.json

@@ -386,5 +386,35 @@
                 }
             }
         }
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_ENABLED": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "synchronous_mode": "enable",
+                    "suppress-fib-pending": "enabled"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_DISABLED": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "synchronous_mode": "disable",
+                    "suppress-fib-pending": "disabled"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_SUPPRESS_PENDING_FIB_ENABLED_SYNCHRONOUS_MODE_DISABLED": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "synchronous_mode": "disable",
+                    "suppress-fib-pending": "enabled"
+                }
+            }
+        }
     }
 }

src/sonic-yang-models/yang-models/sonic-device_metadata.yang

@@ -230,6 +230,15 @@ module sonic-device_metadata {
                                  otherwise the maximum available buffers (which are read from SAI) will be
                                  created, regardless of the CONFIG_DB buffers configuration.";
                 }
+
+                leaf suppress-fib-pending {
+                    type stypes:admin_mode;
+                    default disabled;
+                    description "Enable BGP suppress FIB pending feature. BGP will wait for route FIB installation before announcing routes";
+                    must "((current() = 'disabled') or (current() = 'enabled' and ../synchronous_mode = 'enable'))" {
+                        error-message "ASIC synchronous mode must be enabled in order to enable suppress FIB pending feature";
+                    }
+                }
             }
             /* end of container localhost */
         }