feat(gen): add dart and json output languages to gen types

[spydon]

Summary

Adds two --lang values to supabase gen types, making supabase gen types --lang dart work end to end:

• json: passes the language-neutral GeneratorMetadata introspection document (the @supabase/postgrest-typegen contract) straight through (the same catalog introspection the typescript/go/swift/python generators consume, with exact nullability, defaults, and identity information). This gives third-party and community type generators a single high-fidelity source instead of parsing lossy API descriptions.
• dart: runs the pg-meta container with the json generator, collects the metadata instead of printing it, and pipes it through dart run supabase_typegen --input - --output - on the host. The generated Dart code arrives on stdout like every other language; the typegen's summary line goes to stderr. Requires the Dart SDK on PATH and the supabase_typegen package as a dev dependency of the current project; a missing SDK or failing typegen produces an actionable error.

Both languages work on every connection path (--local, --db-url, --linked, --project-id, and the implicit linked fallback), since non-TypeScript languages already run pg-meta directly.

Draft because of upstream dependencies:

• The postgrest-typegen extraction (feat(postgrest-typegen): add package with multi-language code generation pg-toolbelt#302) defines the GeneratorMetadata contract, and feat: consume @supabase/postgrest-typegen for type generation postgres-meta#1084 makes postgres-meta consume that package; the json value for PG_META_GENERATE_TYPES is added on top of it in feat: add json output exposing the GeneratorMetadata contract postgres-meta#1110 (stacked on fix: bump edge-runtime to v1.2.19 #1084; It should be possible to run npx supabase init without human interaction #1106 was closed as a duplicate of that stack), and this PR needs a pg-meta image that ships it.
• feat: add supabase_typegen package generating typed table definitions supabase-flutter#1635 adds the supabase_typegen package (its --input -/--output - stdin/stdout contract is already in place there).

Implementation notes:

• The lang additions are TS-only and documented in docs/go-cli-divergences.md; SIDE_EFFECTS.md covers the new dart subprocess and the json-collection behavior.
• The invocation contract (dart run supabase_typegen --input - --output -) is pinned in types.shared.ts so the handler and tests share one definition.
• Integration tests cover the json passthrough env, the dart pipe (metadata collected rather than printed, dart argv, generated code on stdout), and the non-zero typegen exit error.

Linked issue

Closes #

• The linked issue is open and carries the open-for-contribution label (or I'm a Supabase maintainer).

Checklist

• The PR title follows Conventional Commits (e.g. fix(cli): …).
• Tests added or updated for the change.
• pnpm check:all and pnpm test pass for the workspace(s) I touched (check:all fully; unit and integration suites for gen types pass, the docker-dependent e2e suite was not run locally).