Add MergeLeafTypes helper and export leaf types

[cochinescu]

Summary

• export generated leaf types so concrete schema leaves can be referenced publicly
• move MergeLeafTypes<> and its internal helper machinery into schema-dts-lib
• have generated schema output import/export MergeLeafTypes from schema-dts-lib
• add consumer compile coverage and update docs

Notes

• this is intentionally additive: existing exported union aliases like Thing and Product are unchanged
• the leaf export change applies to both the standard class leaf declarations and the separate RoleBuiltin leaf type-alias path
• WithContext, Graph, and SchemaValue remain in generator helper output; MergeLeafTypes<> lives in schema-dts-lib with the other schema-agnostic helpers
• most of the diff is expected generator snapshot churn from the new import/export stanza plus public *Leaf declarations

Verification

• NODE_OPTIONS=--experimental-vm-modules ./node_modules/.bin/jest packages/schema-dts-gen/test --runInBand -u
• ./node_modules/.bin/tsc -b packages/schema-dts-lib
• ./node_modules/.bin/tsc -b packages/schema-dts-gen/src
• node packages/schema-dts-gen/dist/src/cli/cli.js > packages/schema-dts/lib/schema.ts
• ./node_modules/.bin/tsc -p packages/schema-dts
• ./node_modules/.bin/tsc -p packages/schema-dts/test
• npm run fix:prettier

Fixes #203

[Eyas]

Random thought (let me know if this is too much of a hassle and I can do part of the change and have you rebase on top of it): I wonder if we can separate the utility types into their own package (e.g. schema-dts-helpers @schema-dts/helpers or similar) and then have the emitted code start with:

import type { WithContext, Graph, MergeLeafTypes } from "schema-dts-helpers";
export {WithContext, Graph, MergeLeafTypes};

instead of the long stanza in each file.

It'll make the snapshot tests & churn easier to read, and it means that we can write things the way we want instead of relying on imperatively constructing the AST with codegen.

Let me know if you'd like me to make the initial structure for that.

[Eyas]

top-level npm test is currently blocked by an unrelated existing lint error in packages/schema-dts/test/withactionconstraints.ts

That's not actually an error, but a problem with the testing -- looks like you need to actively rerun npm run build (of schema-dts-gen at least) before the schema-dts tests work properly.

[cochinescu]

Thanks, this makes sense.

I’m happy to rework the PR in that direction. If you want to sketch the initial helper-package structure, I can rebase on top of it; otherwise I can take a first pass at wiring it up here.

Also thanks for the note on testing — I’ll rerun the schema-dts tests with the required build step and update the PR description accordingly.

[cochinescu]

Rebased this PR onto #213 and moved MergeLeafTypes into schema-dts-lib, while keeping the leaf export change in the generator. I also reran the generator snapshots and the schema-dts build/typecheck flow on top of the new split.

[Eyas]

I think this overload is deprecated, any reason you used it instead of the existing code that passed SyntaxKind.TypeKeyword?

[cochinescu]

Switched this back to SyntaxKind.TypeKeyword to match #213; the boolean overload change was accidental during the split.

[Eyas]

Can you also run:

npm run fix:prettier

to update the formatting of the .ts and .md files?

[cochinescu]

Sure, ran npm run fix:prettier and pushed the formatting updates.

[Eyas]

This is in https://github.com/google/schema-dts/releases/tag/v2.0.0