lnwallet: fix off-by-one error in dust detection for co-op closes

Change Description

This PR changes CreateCooperativeCloseTx to only include a balance output if the output is strictly greater than the dust limit. This matches the behavior of LocalBalanceDust and RemoteBalanceDust, which mark a balance output as dust if it's less than or equal to the dust limit.

Without this change, CreateCooperativeCloseTx includes the balance output when it's greater than or equal to the dust limit. When the balance of the channel responder is exactly the dust limit in a tweakless channel, the transaction includes the output but not the fee for the output, determined by the channel closer in initFeeBaseline here.

When deciding whether to change CreateCooperativeCloseTx or (Local|Remote)BalanceDust, I checked how LDK implements this because this caused a force-close when attempting a co-op close from LND of a channel with LDK. LDK omits the output when it's less than or equal to the dust limit, similar to (Local|Remote)BalanceDust, and does not accept a co-op close transaction with the output included.

Steps to Test

There are a couple of ways I can add a test. The easier way is to directly test for inclusion/exclusion of an output that's exactly the dust amount when calling this function.

A more interesting way would be to create a channel, set the balance on one side to dust, use a ChanCloser to create a close TX, and ensure that the fee rate, total fee, and weight of the transaction match, though it's more involved in terms of setup to provide access to both lnwallet internals and ChanCloser without import cycles.

Pull Request Checklist

Testing

[x] Your PR passes all CI checks.
[ ] Tests covering the positive and negative (error paths) are included.
[ ] Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

[x] The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
[x] Commits follow the Ideal Git Commit Structure.
[x] Any new logging statements use an appropriate subsystem and logging level.
[x] Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
[ ] There is a change description in the release notes, or [skip ci] in the commit message for small changes.

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lightningnetwork / lnd