fix(G118): treat returned cancel func as called (fixes #1584)

[ccojocar]

Summary

Fixes #1584

The isCancelCalled BFS in the G118 (context propagation) analyzer did not handle *ssa.Return, so a cancel function returned from a helper was flagged as "lost" even though responsibility was transferred to the caller.

Root Cause

When a function like initDatabase calls context.WithCancel(ctx) and returns the cancel func to its caller, the SSA representation produces a Return instruction referencing the cancel value. The BFS in isCancelCalled had no case *ssa.Return: branch, so it dead-ended and concluded the cancel was never called.

Changes

• analyzers/context_propagation.go: Add a *ssa.Return case to the isCancelCalled BFS — when the cancel value appears among the return operands, treat it as called (responsibility transferred to caller).
• testutils/g118_samples.go: Update the existing "cancel returned to caller" test to expect 0 issues. Add a new test case matching the exact issue bug: false positive on G118 #1584 pattern (cancel returned as func() and invoked by caller through a shutdown chain).

Validation

• Build: clean
• All 156 analyzer specs pass
• Reproduction case from issue bug: false positive on G118 #1584: 0 issues (was 1)
• Genuinely lost cancel functions: still correctly detected

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (fa74dd7) to head (daef2cd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1585   +/-   ##
=======================================
  Coverage   80.71%   80.72%           
=======================================
  Files         104      104           
  Lines        9878     9882    +4     
=======================================
+ Hits         7973     7977    +4     
  Misses       1418     1418           
  Partials      487      487           

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