feature: allow custom feature bits already set by default

[Lrifton92]

Change Description

Fixes #10883.

When a custom feature bit configured via protocol.custom-init or protocol.custom-nodeann is already advertised in lnd's default feature vector, newManager returned a fatal feature bit: X already set error and lnd failed to start:

unable to create server: feature bit: 39 already set

For example, bit 39 is OnionMessagesOptional, which lnd already advertises by default. A config such as:

[protocol]
protocol.custom-nodeann=39
protocol.custom-init=39

does not change the effective feature vector at all, yet it prevented startup.

Fix

Re-advertising an already-set bit yields an identical feature vector, so it is harmless. This treats an already-set custom bit as a no-op (continue) instead of a fatal error. Genuine conflicts between the optional and required variants of a feature are still rejected by SafeSet immediately below, so the safety property is preserved.

 if raw.IsSet(custom) {
-	return nil, fmt.Errorf("feature bit: %v "+
-		"already set", custom)
+	continue
 }

Testing

Added TestManagerCustomFeatureAlreadyDefault, which configures a custom feature bit that is already set by default and asserts that:

• newManager no longer errors, and
• the resulting raw vector keeps the required bit set without introducing an invalid optional/required pairing.

$ go test ./feature/
ok  	github.com/lightningnetwork/lnd/feature

gofmt, go vet ./feature/ clean. Tested on go1.26.4.

Steps to Test

1. Start lnd with protocol.custom-init=39 and protocol.custom-nodeann=39 in lnd.conf.
2. Before this change lnd aborts with feature bit: 39 already set; after it, lnd starts normally and the feature vector is unchanged.

Pull Request Checklist

• Change is reasonably small and self-contained.
• Tests added/updated and passing.
• Release notes (added in a follow-up commit referencing this PR number).

[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 addresses an issue where lnd would fail to start if a user configured a custom feature bit that was already included in the default feature vector. By allowing these redundant configurations to be ignored, the system remains robust while preventing unnecessary startup aborts.

Highlights

• Error Handling Update: Modified the feature manager to treat custom feature bits that are already set by default as a no-op instead of returning a fatal error.
• Regression Testing: Added a new test case, TestManagerCustomFeatureAlreadyDefault, to ensure that configuring pre-existing feature bits does not cause startup failures or invalid feature vector states.

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[Lrifton92]

Added the release note and contributor entry in c5b26b8 (docs/release-notes/release-notes-0.22.0.md).

[gemini-code-assist]

Code Review

This pull request modifies the feature manager to treat duplicate custom feature bits that are already advertised by default as a no-op instead of failing startup with an error. It also adds a corresponding unit test TestManagerCustomFeatureAlreadyDefault to verify this behavior. There are no review comments, so I have no feedback to provide.

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[github-actions]

PR Severity: HIGH (feature/* - Feature bit management). Files: feature/manager.go [HIGH], docs/release-notes/release-notes-0.22.0.md [LOW], feature/manager_internal_test.go [LOW/excluded]. Scope: 2 non-test files, 17 lines - no bump triggered. <!-- pr-severity-bot -->