junos: add lab for BGP disable-linklocal-addr

[dhalperi]

Two-node vJunos-router 25.4R1.12 lab for the undocumented
disable-linklocal-addr statement. It is hidden from set protocols bgp ? but accepted at the protocol, group, and neighbor levels
(including internal groups and routing instances); it takes no
argument and is rejected under family inet6 and routing-options.
Without it, IPv6 UPDATEs carry a 32-byte MP_REACH next hop (global
plus link-local); with it at any level, only the 16-byte global
address. The receiver's installed route is the same either way, and
committing the statement sends no UPDATE until the next refresh.
The README includes the raw traceoptions, tcpdump, and CLI output.

lab_builder: add a bgp_update_next_hops check that route-refreshes a
Junos neighbor and parses the receiver's BGP update trace.
_junos_commit_check now also treats a bare syntax error. response
to a set line as a rejection; the error: match added in
#235 missed it for unknown keywords.

---

Prompt:

• In batfish/lab-validation, create a public, anonymized Junos lab for
  `protocols bgp disable-linklocal-addr`.

  First use `commit check` on vJunos-router 25.4R1.12 to determine exactly
  where Junos accepts the statement: protocol, group, and neighbor
  hierarchies. Record rejected forms as well as accepted forms.

  If accepted, build a minimal two-router IPv6 eBGP topology. Advertise an
  IPv6 prefix and compare the sender's advertised route and receiver's
  installed route with and without `disable-linklocal-addr`. Capture enough
  raw Junos output to establish its effect on global and link-local BGP
  next-hop addresses.

  Document the Junos version, configuration, commands, and observed
  results. Add automated checks for the supported hierarchy and behavior.
  Do not change Batfish. Commit the lab and open a lab-validation PR.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.59%. Comparing base (2c85390) to head (46decbc).

Additional details and impacted files

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  Coverage   83.59%   83.59%           
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  Files          96       96           
  Lines        4705     4705           
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  Hits         3933     3933           
  Misses        772      772           

Flag	Coverage Δ
unittests	83.59% <ø> (ø)

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