fix(test): make immutable deadline check more conservative

[rvagg]

Fixes: #13098

[BigLep]

@aarshkshah1992 @ZenGround0 : please take a look so we can get this merged.

[Copilot]

Pull Request Overview

This PR updates the immutable deadline check to a more conservative approach, adjusting both the function's logic and its naming.

• Changed the immutable deadline check from an equality comparison to a range-based comparison (current, next, and the one after next)
• Renamed IsImmutableDeadline to MaybeImmutableDeadline to better reflect the updated check

Comments suppressed due to low confidence (2)

itests/kit/node_unmanaged.go:1708

• [nitpick] The function documentation mentions that immutable deadlines are defined as the current or the next deadline, yet the logic now covers three deadlines. Consider updating the comment to clearly reflect that the check now includes the deadline after the next.

// Deadline after the next deadline included to be conservative to avoid race conditions before messages land.

itests/kit/node_unmanaged.go:806

• [nitpick] Renaming the function from IsImmutableDeadline to MaybeImmutableDeadline modifies how the intent is conveyed. Please verify that all call sites and tests are updated accordingly to maintain consistency.

if tm.MaybeImmutableDeadline(provingDeadline) {

[ZenGround0]

Ok this actually makes sense given how the test works. The fact that this method is unused anywhere else and the behavior is so specific to this test makes me suspect its going to be cleaner to just put MaybeImmutableDeadline inline.