convert: lazily create ConstGroup in createEnumItems to avoid empty const nodes

[fennoai]

Requested by @luoliwoshang

Closes #643

Summary

• Lazily initialize ConstGroup in createEnumItems so that p.NewConstGroup() is only called when an enum item is actually emitted
• If all enum items are skipped (e.g., due to name conflicts or filtering), no empty const group is created
• This avoids unnecessary empty const () nodes in generated output

Changes

• cl/internal/convert/package.go: Replaced eager defs := p.NewConstGroup() with a lazy ensureDefs() closure that creates the group on first use

[luoliwoshang]

@xgopilot make a unit_test to mock this behavior will not create a empty const when have not valid enum item, you can make it at /cl/internal/convert/package_bulitin_test.go!

[fennoai]

Added unit test TestNoEmptyConstGroupWhenAllEnumItemsSkipped in cl/internal/convert/package_bulitin_test.go.

What the test does

1. Creates a package and registers enum items (Red, Green) via a first NewEnumTypeDecl("Color", ...) call
2. Calls NewEnumTypeDecl("Color2", ...) with the same items — all items are already registered, so all are skipped in createEnumItems
3. Captures the generated output via WriteTo and asserts there is exactly 1 const ( block (from the first enum), with no empty const () from the second enum

Verification

• go test -run TestNoEmptyConstGroupWhenAllEnumItemsSkipped — PASS
• go fmt and go vet — clean
• Pre-existing test failures (TestFromTestdata, TestDepWithVersion, TestSysToPkg, TestDepPkg) are unrelated and require install.sh

Commit: 3dfc858

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.77%. Comparing base (26d0726) to head (845eb48).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   84.74%   84.77%   +0.02%     
==========================================
  Files          27       27              
  Lines        2314     2318       +4     
==========================================
+ Hits         1961     1965       +4     
  Misses        310      310              
  Partials       43       43              

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[luoliwoshang]

@xgopilot also need check the const Decl not have the another at pkg corresponding ast.File , cover this behavior!

[fennoai]

Enhanced the test to also verify at the Go AST level that no empty const GenDecl exists in the package's ast.File.

What was added

After the existing strings.Count check on the text output, the test now also:

1. Gets the Go AST file via pkg.p.ASTFile("temp.go")
2. Iterates over all declarations and checks that no *goast.GenDecl with Tok == token.CONST has empty Specs
3. Asserts there is exactly 1 const GenDecl in the AST (from the first enum only)

Verification

• go test -run TestNoEmptyConstGroupWhenAllEnumItemsSkipped — PASS
• go fmt and go vet — clean

Commit: 845eb48

[luoliwoshang]

LGTM