graph: fix zombie chan pruning

[Abdulkbk]

Change Description

Fixes #10223

This PR fixes the issue where channels with no edge policies escape pruning even though they have been stale for a while.

The fix only affects native SQL backend see why.

In the PR, we modify the SQL query to return channels without an edge policy, and then we check the timestamp of the block that includes the channel's funding transaction to estimate its age and determine if it should be pruned.

Steps to Test

cd graph && go test  -run  TestIsZombieByAge

cd graph/db && go test -tags test_db_sqlite -run TestChanUpdatesInHorizonWithNoPolicies

[gemini-code-assist[bot]]

Summary of Changes

Hello @Abdulkbk, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where Lightning Network channels that never received an edge policy update were not being properly pruned as 'zombie' channels. The solution introduces a mechanism to determine the staleness of such channels by examining the timestamp of their funding block. This ensures that all inactive channels, regardless of their policy update status, are correctly identified and removed from the graph, leading to a more accurate and efficient network graph representation, specifically for the native SQL backend.

Highlights

• Zombie Channel Pruning Logic: Introduced a new function, IsZombieByAge, to determine if a channel without any edge policies should be considered a 'zombie' based on the timestamp of its funding transaction's block. This addresses a specific issue where such channels were escaping the pruning process.
• Integration into Pruning Process: The pruneZombieChans method has been updated to utilize the new IsZombieByAge logic for channels where both edge policies are absent. This ensures that stale channels lacking policy updates are now correctly identified and added to the list of channels to be pruned.
• SQL Query Modification: The ChanUpdatesInHorizon SQL query was modified to explicitly include channels that have no associated edge policies (i.e., both last_update timestamps are NULL). This change is crucial for the SQL backend to correctly retrieve these channels for age-based zombie detection and pruning.
• New Test Cases: Added TestIsZombieByAge to graph/builder_test.go to validate the new age-based zombie detection logic, and TestChanUpdatesInHorizonWithNoPolicies to graph/db/graph_test.go to ensure the SQL backend correctly returns channels without policies within various time horizons.

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[lightninglabs-deploy]

@ziggie1984: review reminder @abdulkbk, remember to re-request review from reviewers when ready

[Abdulkbk]

Thanks for taking a look @MPins 🙏