fix(G118): eliminate false positive when cancel is called via struct field in a closure

[ravisastryk]

Fix G118 false positive when cancel is called via struct field in a closure

Fixes #1595

Problem

G118 fires a false positive when a cancel function is stored into a struct field after construction and later invoked through that field inside a deferred closure:

foo.cancel = cancel
defer func() { foo.cancel() }()

The existing fix from #1593 handles the constructor + return pattern but not this one. Here the struct is never returned — the call lives in a closure, which SSA compiles into a separate *ssa.Function. The isCancelCalled BFS traces referrers of the FieldAddr from the Store, but that value has no referrers bridging to the closure’s own FieldAddr, so the call is never seen.

Fix

Adds isFieldCalledInAnyFunc which scans all SSA functions (including closures) for a FieldAddr matching the same struct pointer type and field index. If any such address is loaded and used as a call or defer target, the cancel is considered handled.

Called from the Store+FieldAddr branch in isCancelCalled, after the existing isStructFieldReturnedFromFunc check.

Testing

• Added test samples for the exact pattern from the issue (deferred closure) and the direct-call variant
• Verified genuinely lost cancels (field assigned but never called) still trigger G118
• All existing G118 tests pass

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.90%. Comparing base (619ce21) to head (dc5511e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
analyzers/context_propagation.go	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1596      +/-   ##
==========================================
+ Coverage   80.89%   80.90%   +0.01%     
==========================================
  Files         104      104              
  Lines        9929     9947      +18     
==========================================
+ Hits         8032     8048      +16     
- Misses       1407     1408       +1     
- Partials      490      491       +1     

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