fix: track nil flows through function variables

[yd-beep]

Previously, nilaway would skip nil analysis for calls made through function variables (e.g., var ProcessFunc = process; ProcessFunc(nil)), treating them as trusted non-nil. This caused false negatives where nil pointer dereferences were not detected.

This change adds support for resolving function variables to their underlying function declarations, enabling proper nil flow tracking through both return values and arguments.

Changes:

• Add resolveFuncVariable() helper to resolve function variables to their underlying *types.Func for both package-level declarations (var F = f) and local assignments (f := someFunc)
• Update ParseExprAsProducer() to use resolved functions for return value analysis when calling through function variables
• Update AddComputation() to use resolved functions for argument consumption tracking when calling through function variables
• Add getFuncReturnProducersForFunc() to support direct *types.Func usage without requiring an *ast.Ident
• Add integration tests for function variable nil tracking

If resolution fails (dynamic assignment, closures, etc.), the code falls back to the previous trusted behavior to avoid false positives.

Fixes #387

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.01%. Comparing base (89df5f7) to head (4befe05).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tion/function/assertiontree/root_assertion_node.go	85.71%	2 Missing and 2 partials ⚠️
...tion/function/assertiontree/parse_expr_producer.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
- Coverage   87.01%   87.01%   -0.01%     
==========================================
  Files          73       73              
  Lines        8305     8340      +35     
==========================================
+ Hits         7227     7257      +30     
- Misses        885      888       +3     
- Partials      193      195       +2     

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