Improve HTLC interceptor setup handling and fix itest flake

[ffranr]

This PR strengthens tapd startup guarantees and fixes the flake in the unknown_TLV_type itest.

Key points

• HTLC interceptor setup failures now cause tapd to exit rather than continue in a degraded state. This ensures that a running tapd instance always has a functioning RFQ subsystem.
• The itest flake was caused by two tapd instances sharing the same lnd backend, competing for a single HTLC interceptor. The test now gives Charlie its own lnd node, eliminating the conflict.

[gemini-code-assist]

Summary of Changes

Hello @ffranr, 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 significantly enhances the stability and reliability of the tapd daemon by ensuring that critical components, specifically the HTLC interceptor, are properly initialized. If setup fails, tapd will now gracefully exit rather than operating in a compromised state. Additionally, it addresses a persistent flake in an integration test by isolating lnd backend resources, preventing resource contention and improving test determinism.

Highlights

• Robust HTLC Interceptor Setup: HTLC interceptor setup failures will now cause tapd to exit, preventing it from running in a degraded state and ensuring the Request-for-Quote (RFQ) subsystem is always functional.
• Integration Test Flake Resolution: The unknown_TLV_type integration test flake has been fixed by providing a dedicated lnd backend for the 'Charlie' tapd instance, resolving a conflict where two tapd instances were competing for a single HTLC interceptor.

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[gemini-code-assist]

Code Review

This pull request strengthens tapd startup guarantees by making HTLC interceptor setup failures critical, which now causes the daemon to exit. This is a good improvement to ensure the RFQ subsystem is always functional when tapd is running. The change is implemented by propagating an error channel to the OrderHandler and reporting the setup failure as a critical error. Additionally, the PR fixes an integration test flake by providing a dedicated lnd node to a tapd instance, which correctly resolves the resource conflict. The changes are well-reasoned and correctly implemented. I have a couple of minor suggestions to improve code comments and style guide adherence.

[ffranr]

This PR should fix this sort of flake:

        tapd_harness.go:336: Using proof courier address: universerpc://127.0.0.1:10259
        test_harness.go:137: Failed: (unknown TLV type): exited with error: 
            *errors.errorString Error running server: unable to initialize RPC server: unable to start RFQ manager: unable to start RFQ order handler: unable to setup incoming HTLC interceptor: rpc error: code = Unknown desc = interceptor already exists
            /home/runner/work/taproot-assets/taproot-assets/itest/tapd_harness.go:458 (0x2505552)
            	(*tapdHarness).start.func2: hs.ht.Fatalf("Error running server: %v", err)
            /opt/hostedtoolcache/go/1.24.9/x64/src/runtime/asm_amd64.s:1700 (0x4823a1)
            	goexit: BYTE	$0x90	// NOP
        test_harness.go:461: 
            	Error Trace:	/home/runner/work/taproot-assets/taproot-assets/itest/test_harness.go:461
            	            				/home/runner/work/taproot-assets/taproot-assets/itest/addrs_test.go:811
            	            				/home/runner/work/taproot-assets/taproot-assets/itest/test_harness.go:160
            	            				/home/runner/work/taproot-assets/taproot-assets/itest/integration_test.go:131
            	Error:      	Received unexpected error:
            	            	error waiting for server to start: dial tcp 127.0.0.1:10354: connect: connection refused
            	Test:       	TestTaprootAssetsDaemon/tranche01/15-of-100/unknown_TLV_type
    harness_miner.go:149: Mined 0 blocks when cleanup the mempool

https://github.com/lightninglabs/taproot-assets/actions/runs/19930326488/job/57140421069

[coveralls]

Pull Request Test Coverage Report for Build 19979510397

Details

• 14 of 16 (87.5%) changed or added relevant lines in 2 files are covered.
• 32 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+0.06%) to 56.658%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rfq/order.go	13	15	86.67%

Files with Coverage Reduction	New Missed Lines	%
asset/group_key.go	2	72.15%
commitment/tap.go	2	85.42%
lndservices/block_header_cache.go	2	97.52%
mssmt/compacted_tree.go	2	78.11%
tapdb/universe.go	2	81.27%
tapgarden/custodian.go	2	77.39%
itest/multisig.go	3	97.94%
tapchannel/aux_leaf_signer.go	3	43.53%
tapdb/multiverse.go	6	80.16%
tapgarden/caretaker.go	8	76.63%

Totals
Change from base Build 19960793390:	0.06%
Covered Lines:	64900
Relevant Lines:	114546

---

💛 - Coveralls

[jtobin]

One question about the new ErrChan, otherwise LGTM. 👍 👍

[jtobin]

🚢 🚢