feat: add supabase_typegen package generating typed table definitions

[spydon]

What kind of change does this PR introduce?

Feature (draft, layer 2 of the typed table access work, stacked on #1634). Adds a new supabase_typegen package: a standalone code generator that turns a database schema into the typed table definitions introduced in #1634, so users get the fully typed surface without writing any of it by hand.

Linear: SDK-1362

What is the new behavior?

dart run supabase_typegen --url https://your-project.supabase.co --key $SUPABASE_ANON_KEY

reads the OpenAPI (Swagger 2.0) description that PostgREST serves at the API root and emits one Dart file containing, per table:

• a zero-cost row extension type over the decoded JSON map with typed getters (DateTime parsing, double/num coercion, List casts, Postgres enum mapping),
• Insert and Update value extension types that implement Map<String, dynamic>, with required parameters derived from NOT NULL-without-default columns and null-aware omission for everything else; explicit SQL NULL writes go through generated set…ToNull copy methods that only exist for nullable columns,
• a PostgrestTable definition plus TableColumn tokens for compile-time checked filters,
• Dart enums for Postgres enums with wire-name mapping (toString returns the wire name so enum values work directly in filters).

See packages/supabase_typegen/test/goldens/supabase_schema.dart for what the output looks like for the fixture schema.

Design choices worth reviewing:

• Introspection source: the PostgREST OpenAPI description. It needs nothing but a URL and an API key, which makes the tool trivially usable against local and hosted projects (the same approach supadart proved viable). The parser is a pure function over the JSON document, so a higher-fidelity pg-meta or direct Postgres backend can be added behind the same SchemaDescription model later.
• Nullability heuristic: OpenAPI cannot distinguish nullable columns from NOT NULL columns with a database default, so getters are non-nullable only for required columns and primary keys; everything else is conservatively nullable. Documented in the README as the main reason to add a pg-meta backend eventually.
• Naming: books emits BooksRow/BooksInsert/BooksUpdate plus a Books namespace class (no English singularization, so names stay predictable). Identifiers are sanitized against Dart reserved words and Map member names with a $ suffix, and collisions are deduplicated.
• Lint-clean output: the emitted code (checked in as a golden) passes supabase_lints and DCM with zero issues, including the strict extension type rules.

Deliberately deferred: foreign key relationship getters (the fk targets are already parsed into the model) and typed rpc functions.

Additional context

• The package is excluded from the SDK compliance scan via .sdk-parse-ignore since it is a development-time tool, not SDK client surface; the symbol, drift and schema checks pass locally against the base branch.
• supabase_typegen is added to the CI dart test matrix; tests are fully mocked/fixture-based (introspection unit tests, a whitespace-insensitive golden comparison with a tool/regenerate_goldens.dart refresh script, and behavior tests that run the generated golden code against a mock HTTP client to verify wire formats end to end).
• publish_to: none until the API settles.

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