Make 'show bfd {summary|peer}' commands work for software BFD sessions

[kalash-nexthop]

fixes# 4139

What I did

When software BFD is enabled (CONFIG_DB:SYSTEM_DEFAULTS|software_bfd|status=enabled), the show bfd summary and show bfd peer commands now:

1. Query FRR directly via vtysh (similar to show ip bgp commands)

2. Filter BFD sessions to only show peers configured in CONFIG_DB:

   • BGP neighbors with bfd=true
   • Static routes with bfd=true

   This fixes the issue where software BFD sessions were not visible in 'show bfd summary' because they are not written to STATE_DB:BFD_SESSION_TABLE.

For hardware BFD, the commands continue to use STATE_DB as before.

How I did it

Changes:

• Added utilities_common/bfd_util.py with helper functions:

  • is_software_bfd_enabled(): Check if software BFD is configured
  • get_bfd_peers_from_config(): Get BFD-enabled peers from CONFIG_DB
  • get_bfd_sessions_from_frr(): Query FRR via vtysh
  • filter_bfd_sessions_by_config(): Filter FRR sessions by CONFIG_DB

• Updated show/main.py:

  • Modified 'show bfd summary' to use vtysh for software BFD
  • Modified 'show bfd peer' to use vtysh for software BFD
  • Both commands filter output to only show configured peers

How to verify it

Unit tests are added.

Manual verification:

vtysh output:
admin@vlab-01:~$ vtysh -c "show bfd peers json" | jq .
[
  {
    "multihop": false,
    "peer": "10.0.0.33",
    "local": "10.0.0.32",
    "vrf": "default",
    "interface": "Ethernet4",
    "id": 3483190008,
    "remote-id": 0,
    "passive-mode": false,
    "status": "down",
    "downtime": 1697,
    "diagnostic": "ok",
    "remote-diagnostic": "ok",
    "type": "dynamic",
    "receive-interval": 300,
    "transmit-interval": 300,
    "echo-receive-interval": 50,
    "echo-transmit-interval": 0,
    "detect-multiplier": 3,
    "remote-receive-interval": 1000,
    "remote-transmit-interval": 1000,
    "remote-echo-receive-interval": 0,
    "remote-detect-multiplier": 3,
    "rtt-min": 0,
    "rtt-avg": 0,
    "rtt-max": 0
  }
]

sonic command output after the change:

admin@vlab-01:~$ show bfd summary
Total number of BFD sessions: 1
Peer Addr    Interface    Vrf      State    Type     Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  -------  ------------  -------------  -------------  ------------  ----------  ---------------------
10.0.0.33    Ethernet4    default  down     dynamic  10.0.0.32               300            300             3  no                     3483190008

admin@vlab-01:~$ show bfd peer 10.0.0.33
Total number of BFD sessions for peer IP 10.0.0.33: 1
Peer Addr    Interface    Vrf      State    Type     Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  -------  ------------  -------------  -------------  ------------  ----------  ---------------------
10.0.0.33    Ethernet4    default  down     dynamic  10.0.0.32               300            300             3  no                     3483190008
admin@vlab-01:~$

Previous command output (if the output of a command-line utility has changed)

admin@vlab-01:~$ show bfd summary
show bfd summary
Total number of BFD sessions: 0
Peer Addr    Interface    Vrf    State    Type    Local Addr    TX Interval    RX Interval    Multiplier    Multihop    Local Discriminator
-----------  -----------  -----  -------  ------  ------------  -------------  -------------  ------------  ----------  ---------------------

admin@vlab-01:~$ show bfd peer 10.0.0.33
No BFD sessions found for peer IP 10.0.0.33
admin@vlab-01:~$

New command output (if the output of a command-line utility has changed)

admin@vlab-01:~$ show bfd summary
Total number of BFD sessions: 1
Peer Addr    Interface    Vrf      State    Type     Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  -------  ------------  -------------  -------------  ------------  ----------  ---------------------
10.0.0.33    Ethernet4    default  down     dynamic  10.0.0.32               300            300             3  no                     3483190008

admin@vlab-01:~$ show bfd peer 10.0.0.33
Total number of BFD sessions for peer IP 10.0.0.33: 1
Peer Addr    Interface    Vrf      State    Type     Local Addr      TX Interval    RX Interval    Multiplier  Multihop      Local Discriminator
-----------  -----------  -------  -------  -------  ------------  -------------  -------------  ------------  ----------  ---------------------
10.0.0.33    Ethernet4    default  down     dynamic  10.0.0.32               300            300             3  no                     3483190008
admin@vlab-01:~$

[mssonicbld]

/azp run

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