feat: support ellipsis placeholder expression

[hjotha]

Summary

Adds ... as a microflow expression placeholder for intentionally unbound code-action parameters. This lets describe output preserve empty BasicCodeActionParameterValue bindings and lets exec write them back without inventing synthetic expression values.

Part of #332.

Changes

• Adds ELLIPSIS to the MDL grammar and preserves it as a source expression.
• Maps Java action arguments written as ... back to empty basic code-action parameter values.
• Documents the syntax in the quick reference, proposal index, and Mendix microflow skill.
• Adds a doctype example and parser/builder regression coverage.

Validation

• make build
• make lint-go
• make test
• ./bin/mxcli check mdl-examples/doctype-tests/ellipsis_placeholder.test.mdl

[github-actions]

AI Code Review

Critical Issues

None found.

Moderate Issues

None found.

Minor Issues

• Visitor logic not explicitly shown: While the implementation appears correct (tests pass and documentation states the visitor produces ast.SourceExpr{Source: "..."} for ELLIPSIS), the diff doesn't show explicit visitor changes. Given the truncation note, this may be in the omitted sections, but it would be good to confirm the visitor correctly bridges the ELLIPSIS token to the SourceExpr AST node.

What Looks Good

• Full-stack implementation: The feature is properly implemented through the entire pipeline:

  • Grammar: Added ELLIPSIS token and rule
  • AST: Added SourceExpr to preserve source text
  • Executor: Handles placeholder detection and conversion to empty BasicCodeActionParameterValue
  • Tests: Includes unit test verifying BSON preservation and doctype example
  • Documentation: Updated quick reference, skills doc, and added detailed proposal

• Correct semantics: The implementation correctly preserves empty Argument values in BSON for round-trip compatibility, addressing the core motivation of maintaining symmetry in describe → exec → describe cycles.

• Clear documentation: The proposal and accompanying documentation clearly explain that ... is round-trip-only and should not be used in newly authored scripts, preventing misuse.

• Robust handling: The implementation correctly handles whitespace variations (e.g., " ... ") through trimming in the placeholder detection function.

• Test coverage: Includes both unit test for the builder logic and a doctype test for end-to-end validation.

• Minimal scope: Focused solely on adding the ellipsis placeholder for Java action parameters without attempting to over-extend to other contexts prematurely.

Recommendation

Approve. The PR thoroughly implements the ellipsis placeholder feature following all project guidelines, with proper documentation, test coverage, and full-stack implementation. The minor concern about visitor logic is likely addressed in the generated or omitted code sections given the passing tests and explicit documentation of the AST behavior.

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