fix(cli): keep doc comments of imported grammar elements when embedding

[simonyang08]

Fixes #2065

Root cause

embedReferencedGrammar (packages/langium-cli/src/generate.ts) copies imported grammar elements with AstUtils.copyAstNode, which drops the `` reference. Both comment consumers resolve comments through the CST:

• the grammar serializer (comments: true → DefaultCommentProvider → findCommentNode(node.$cstNode, ...))
• the BNF generator (findCommentNode(rule.$cstNode, ['ML_COMMENT']))

so rules and types from imported grammars lose their doc comments in the generated grammar.ts (``) and BNF output.

Fix

Re-attach the original CST node to each copied element:

Object.defineProperty(copy, '', { value: element., writable: true });

Embedded grammars are only used for read-only generation, so sharing the CST node with the imported grammar's document is safe. (The defineProperty route is used because `` is typed readonly and copyAstNode deliberately skips `$`-properties.)

Tests

New packages/langium-cli/test/generator/embed-grammars.test.ts drives the real embedGrammars path with a two-grammar setup (import + cross-reference) and locks down:

1. embedded elements keep their ``
2. the BNF output contains the imported rule's doc comment
3. the serialized grammar contains `` for imported rules and types

vitest run packages/langium-cli/test: 57 passed, 0 failed (54 existing + 3 new).

Two adjacent observations from reproducing this

1. Only multiline comments are collected. DefaultCommentProvider follows GrammarConfig.multilineCommentRules (auto-derived from multiline comment terminal rules, so SL_COMMENT//// line docs are invisible by design), and the BNF generator hardcodes ['ML_COMMENT'] instead of going through CommentProvider — so a custom multilineCommentRules override wouldn't reach the BNF output. Worth a deliberate decision at some point; out of scope here.
2. ast.ts never emits JSDoc. We checked the current main and v3.5.0: the AST generator contains no comment emission at all, so the 'JSDoc appears on interfaces but not types' half of the original report doesn't reproduce on official code — the actual comment-loss symptoms are confined to grammar.ts and BNF output, both covered by this fix.

Reproduction project: two grammars (import + cross-reference), bnf target enabled — happy to share if useful.

[msujew]

Thanks, works well 👍