refactor(p2p/discover): improve table addition code #18974

[gzliudan]

Proposed changes

Ref: ethereum#18974

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
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[Copilot]

Pull request overview

This PR refactors the discovery table’s node addition logic to distinguish between “seen” nodes (unverified) and “verified” nodes, improving how nodes are inserted/updated in buckets and updating associated tests accordingly.

Changes:

• Replace the previous generic table add path with addSeenNode (append to bucket / add as replacement) and addVerifiedNode (bump/add to front with init-safety).
• Update bucket bumping to handle endpoint/IP changes while respecting IP limits.
• Adjust discovery UDP ping handling and update/add tests to cover the new behaviors.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
p2p/discover/udp.go	Switch ping handler to the new verified-node insertion path.
p2p/discover/table.go	Introduce addSeenNode/addVerifiedNode, refactor bump logic, and add a shared contains helper.
p2p/discover/table_util_test.go	Update test helpers to use the new add API.
p2p/discover/table_test.go	Update existing tests and add new ones for addSeenNode/addVerifiedNode and IP-limit invariants.