feat(kad): add Behaviour::get_record_from

[fully-autonomous]

Description

Adds Behaviour::get_record_from(key, peers): a seeded GET that contacts only a
caller-supplied peer set and never expands to peers discovered mid-walk. It is the GET
counterpart of the existing Behaviour::put_record_to — where put_record_to writes to a
fixed peer set, get_record_from reads from one. The local store is consulted exactly as in
get_record (a local hit is emitted immediately as GetRecordOk::FoundRecord with peer: None).

Motivation: S/Kademlia §4.2 node-disjoint lookups require issuing d independent lookups, each
restricted to a disjoint seed group, so the paths provably never converge on a shared
intermediate node. get_record always seeds from the routing table and expands iteratively, so
this is not currently possible without forking the crate. get_record_from reuses the existing
QueryPool::add_fixed / FixedPeersIter machinery that already backs put_record_to, whose
on_success never folds in newly discovered peers — making the GET pinned by construction.

Implementation mirrors get_record exactly, substituting queries.add_fixed(peers, info) for
add_iter_closest(...). The change is purely additive: no existing method, signature, or trait
is changed, and QueryPool::add_fixed remains pub(crate).

AI Assistance Disclosure

Tools used (required — write none if no AI was used): Claude Code

Attestation (required):

• I have read every line of this diff, understand what it does, and can explain it in review.

Notes & open questions

Two integration tests were added in protocols/kad/src/behaviour/test.rs, in the style of the
existing get_record* tests:

• get_record_from_seeded_peer — seeding the GET with a peer that holds the record returns it.
• get_record_from_does_not_expand_beyond_seeded_peers — the record lives on an un-seeded
  node that the querying node knows directly; seeding the GET with a different peer that lacks
  the record yields GetRecordError::NotFound and never contacts the record-holder, proving the
  query does not expand beyond the fixed set (the disjointness property the use case relies on).

Open question for maintainers: the changelog entry has been added under the unreleased 0.49.0
section; happy to move it to a different version header if you prefer.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates