junos: add lab for BGP path-selection compare-cluster-length-before-originator-id

[dhalperi]

vJunos-router 25.4R1.12 lab for the undocumented path-selection compare-cluster-length-before-originator-id statement. It is hidden
from set protocols bgp path-selection ? but accepted as a leaf in the
master instance, virtual-router and vrf instances, and logical systems;
path-selection does not exist at the group or neighbor level. dut
receives two reflected iBGP paths for 203.0.113.0/24 that tie until the
originator-ID step: A with a one-entry cluster list and originator
10.255.0.9, B with a two-entry cluster list and originator 10.255.0.1.
Without the statement Junos selects B ("Router ID"); with it, A
("Cluster list length"). Committing or deleting the statement on a
live session changes the selection without a session reset. dut runs
both states at once, in the master instance and a KNOB virtual-router.
The reflectors and originators are cEOS.

Batfish does not parse the statement and selects B in both instances;
test_bgp_rib_routes[dut] is sickbayed to batfish/batfish#10370 and to
#239, since JunosValidator drops Batfish's iBGP
routes in non-default VRFs before matching.

lab_builder: add a bgp_active_path check that asserts a Junos
prefix's active BGP path by originator ID and the inactive reason on
the other paths.

Junos BGP parser: accept the (Originator) flag and the Cluster list: and Originator ID: lines that reflected routes add to the
as-path field.

For batfish/batfish#10370 and #239.

---

Prompt:

• Create a public lab-validation experiment for Junos
  `compare-cluster-length-before-originator-id`.

  Use vJunos-router and anonymized addresses, ASNs, and hostnames. Validate:

  1. Whether this syntax commits:
     set protocols bgp path-selection compare-cluster-length-before-originator-id

  2. Whether it also commits under a routing instance. Check and document
     rejection at plausible unsupported levels such as group and neighbor.

  3. Its route-selection behavior. Advertise two otherwise equivalent iBGP
     routes for the same prefix:
     - one with a shorter cluster list but a larger originator ID
     - one with a longer cluster list but a smaller originator ID

  Arrange the attributes so the originator-ID and cluster-list comparisons
  prefer opposite paths. Record the selected path without the statement,
  then enable the statement and record whether the selected path changes.

  Capture the Junos version, commit results, relevant configuration, received
  BGP attributes, and selected-route output. Explain any lab limitations in
  the README and keep all committed artifacts suitable for a public GitHub
  repository.

Six vJunos-router VMs oversubscribed the lab host's CPU, so only dut
runs Junos and the other nodes are cEOS.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.59%. Comparing base (512d50c) to head (82e751b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #238   +/-   ##
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  Coverage   83.59%   83.59%           
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  Files          96       96           
  Lines        4705     4706    +1     
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+ Hits         3933     3934    +1     
  Misses        772      772           

Flag	Coverage Δ
unittests	83.59% <100.00%> (+<0.01%)	⬆️

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