itest: add chancloser aux output regression coverage

[darioAnongba]

• added a custom-channels integration test that reproduces the coop-close fee baseline bug for asset channels with auxiliary close outputs
• added a low-fee coop-close test helper so the test can explicitly close at relay floor
• narrowed the assertion to the behavior this regression cares about: the cooperative close completes and includes the expected auxiliary output

Why

lnd PR #10615 fixes coop-close fee baseline calculation for channels whose final close transaction includes auxiliary outputs. Before that fix, the initial taproot/custom-channel close offer could be underpriced, causing the responder to reject it with unable to process close msg: insufficient fee.

This PR adds taproot-assets itest coverage for that exact path so we can verify the upstream fix end to end.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! 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 adds a deterministic regression test to the custom-channels integration suite. The purpose is to validate the fix for a cooperative close fee baseline bug that occurred when channels included auxiliary outputs. By simulating a close at the relay fee floor, the test confirms that the fee estimation logic correctly accounts for these outputs, ensuring successful channel closure.

Highlights

• Regression Test Added: Introduced a new integration test case to verify cooperative close fee estimation for channels with auxiliary outputs.
• New Test Helper: Added a helper function to facilitate closing asset channels at specific fee rates, enabling testing at the relay floor.
• Validation: The test ensures that cooperative closes succeed even when auxiliary outputs are present, preventing previous underpricing issues.

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[gemini-code-assist]

Code Review

This pull request introduces a regression test for cooperative close fee estimation when auxiliary outputs are present, ensuring that closing at the relay floor remains successful. It also adds helper functions closeAssetChannelWithFeeAndAssert and waitForClosePendingUpdate to facilitate testing channel closures with specific fee rates. Feedback suggests replacing the magic number 1000 with a named constant for better maintainability and improving the reliability of channel closure operations by using a longer timeout and applying it to the CloseChannel RPC call.

[coveralls]

Coverage Report for CI Build 24788235266

Coverage decreased (-0.005%) to 34.364%

Details

• Coverage decreased (-0.005%) from the base build.
• Patch coverage: 91 uncovered changes across 1 file (0 of 91 lines covered, 0.0%).
• 28 coverage regressions across 5 files.

Uncovered Changes

File	Changed	Covered	%
itest/custom_channels/helpers.go	91	0	0.0%

Coverage Regressions

28 previously-covered lines in 5 files lost coverage.

File	Lines Losing Coverage	Coverage
authmailbox/server.go	9	76.39%
tapgarden/caretaker.go	8	68.08%
tapchannel/aux_leaf_signer.go	5	43.18%
tapdb/universe.go	4	76.99%
asset/group_key.go	2	62.64%

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Coverage Stats

Relevant Lines:	100263
Covered Lines:	34454
Line Coverage:	34.36%
Coverage Strength:	0.37 hits per line

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💛 - Coveralls