fix: return new BitList instance from emptyList() to prevent shared mutable state

[daguimu]

Problem

BitList.emptyList() returns a shared static singleton instance. Since BitList is mutable (supports add(), addToTailList(), and(), etc.), any caller that modifies the returned "empty" list inadvertently mutates the singleton, contaminating all subsequent callers.

In AbstractDirectory, the invokers field is initialized with BitList.emptyList(). When the first interface's invokers are added, they pollute the shared singleton. When a second interface later calls BitList.emptyList(), it receives a list already containing the first interface's invokers, causing traffic to be routed to the wrong instances and resulting in NoSuchMethodError.

Root Cause

// Shared mutable singleton - any mutation affects all callers
private static final BitList emptyList = new BitList(Collections.emptyList());

public static <T> BitList<T> emptyList() {
    return emptyList; // returns the SAME instance every time
}

Fix

Return a new BitList instance on each call to emptyList(), consistent with how mutable collections should behave:

public static <T> BitList<T> emptyList() {
    return new BitList<>(Collections.emptyList());
}

Tests Added

• testEmptyListReturnsIndependentInstances — Verifies each call returns a distinct object
• testEmptyListMutationDoesNotAffectOtherEmptyList — Reproduces the exact bug: adding elements to one empty list must not affect another (the core scenario from [Bug] The traffic will be routed to instances of previous interface, which causes no such method error #16131)
• testEmptyListTailListMutationDoesNotAffectOtherEmptyList — Verifies tail list mutations are also isolated
• testEmptyListIsInitiallyEmpty — Confirms the returned list starts truly empty

All 26 tests in BitListTest pass.

Impact

• Minimal change: removes 1 static field, modifies 1 method (3 lines changed in production code)
• No behavioral change for callers — they get the same empty BitList, just a fresh instance
• No reference equality (==) checks exist against BitList.emptyList() in the codebase
• Fixes a critical production bug where cross-interface invoker leakage causes NoSuchMethodError

Fixes #16131

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.83%. Comparing base (63fe8c3) to head (c862f97).
⚠️ Report is 1 commits behind head on 3.3.

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16169      +/-   ##
============================================
+ Coverage     60.81%   60.83%   +0.02%     
+ Complexity    11765    11752      -13     
============================================
  Files          1953     1953              
  Lines         89118    89117       -1     
  Branches      13444    13444              
============================================
+ Hits          54197    54217      +20     
+ Misses        29364    29347      -17     
+ Partials       5557     5553       -4     

Flag	Coverage Δ
integration-tests-java21	32.17% <100.00%> (+<0.01%)	⬆️
integration-tests-java8	32.22% <100.00%> (-0.09%)	⬇️
samples-tests-java21	32.12% <100.00%> (-0.07%)	⬇️
samples-tests-java8	29.72% <100.00%> (-0.04%)	⬇️
unit-tests-java11	59.03% <100.00%> (-0.01%)	⬇️
unit-tests-java17	58.53% <100.00%> (-0.01%)	⬇️
unit-tests-java21	58.51% <100.00%> (-0.03%)	⬇️
unit-tests-java25	58.50% <100.00%> (-0.02%)	⬇️
unit-tests-java8	59.04% <100.00%> (+0.01%)	⬆️

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