fix: track conditional reassignments in errgroup/spawner checkers

[mpyw]

Summary

• Extended conditional reassignment fix from fix: track conditional reassignments for proper context propagation check #22 to errgroup, waitgroup, conc, and spawner checkers
• Added FuncLitAssignment type and FuncLitAssignmentsOfIdent method to probe package
• Both SpawnCallbackChecker and SpawnerChecker now properly track all assignments

Problem

Same issue as the goroutine checker: when a variable is conditionally reassigned:

func bad(ctx context.Context) {
    g := new(errgroup.Group)
    fn := func() error {
        fmt.Println("no ctx")
        return nil
    }
    if condition {
        fn = func() error {
            _ = ctx
            return nil
        }
    }
    g.Go(fn)  // No warning - but should warn!
}

The analyzer only checked the last assignment and would incorrectly pass if the conditional branch used context while the unconditional assignment didn't.

Solution

• Added FuncLitAssignment struct with Conditional flag
• Use inspector.WithStack to detect if assignment is inside control structure
• Check ALL assignments from last unconditional onwards
• ALL must capture context for the check to pass

Test plan

• All existing tests pass
• Added 3 new test cases for conditional reassignment patterns in errgroup
  • badConditionalReassignFirstUsesCtx: First uses ctx, conditional doesn't → should warn
  • badConditionalReassignFirstNoCtx: First doesn't use ctx, conditional does → should warn
  • goodConditionalReassignAllUseCtx: All assignments use ctx → should NOT warn

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.18%. Comparing base (1069525) to head (0e6eafc).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   83.96%   84.18%   +0.22%     
==========================================
  Files          24       24              
  Lines        1808     1834      +26     
==========================================
+ Hits         1518     1544      +26     
  Misses        173      173              
  Partials      117      117              

Flag	Coverage Δ
unittests	84.18% <100.00%> (+0.22%)	⬆️

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