test: kad integration tests

[rlve]

Depends on: vacp2p/nim-libp2p#2149

[Copilot]

Pull request overview

Adds a dedicated QtTest integration test executable for Kademlia (Kad) behaviors, separating Kad scenarios out of the existing integration suite and updating the Nix flake lock to a newer nim-libp2p revision (per the dependency noted in the PR description).

Changes:

• Introduce tests/integration_kad.cpp with integration coverage for Kad put/get, find-node, providing lifecycle, and random-records.
• Remove the older kadPutGet() scenario from tests/integration.cpp (now moved/refined into the new Kad-focused test file).
• Update flake.lock to pin nim-libp2p to a newer commit.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 6 comments.

File	Description
tests/integration_kad.cpp	New Kad-focused integration test executable covering multiple Kad operations.
tests/integration.cpp	Removes the prior inline Kad put/get integration test.
flake.lock	Bumps pinned nim-libp2p revision used by Nix builds.

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[gmelodie]

@rlve I merged test-3 before this one, can you point this PR to master?