fix: escape identifiers in the Python type generator - #1082

Enum labels and column aliases were interpolated into the generated
Python source unescaped, so a name containing a double quote, backslash
or newline produced invalid Python and could inject text into the
generated module. Emit them through JSON string escaping, which is a
subset of Python's string literal escaping.

Ported from supabase/postgres-meta#1082

…columns, composite nullability (#124)

## Summary

Ports the worthwhile Python generator fixes from postgres-meta's open
template PRs into this package (the templates are being deleted in favor
of this package in supabase/postgres-meta#1084, so open fixes there are
triaged and re-landed here). Four fixes, one commit each:

1. **Identifier escaping** (from supabase/postgres-meta#1082): enum
`Literal` labels and `Field(alias=...)` values were interpolated
unescaped, so a quote, backslash, or newline in a database name broke
the generated module. A shared `escapePythonString` helper (JSON
escaping, a strict subset of Python's) now covers all three
interpolation sites.
2. **Python 3.9/3.10 support** (from supabase/postgres-meta#1094):
`NotRequired` (3.11+) and `TypeAlias` (3.10+) now import from
`typing_extensions`, which is always installed as a required dependency
of pydantic.
3. **Deserialized json/jsonb** (from supabase/postgres-meta#1129):
`json`/`jsonb` map to pydantic's `JsonValue` instead of `Json[Any]`.
PostgREST returns these columns already deserialized, while `Json[Any]`
validates a JSON *string* and parses it, so every generated model with a
JSON column failed `model_validate` (supabase/supabase-py#1597).
4. **Composite type nullability** (the Python side of
supabase/postgres-meta#1063, reimplemented): composite type attributes
cannot carry NOT NULL constraints in Postgres, so their fields now emit
`Optional[...]`. The origin PR's Python hunks were dead code (an unused
`PythonDomain` class and a type-map entry for a name Postgres never
emits), so the actual fix was implemented instead of ported.

## Triage of origin PRs

| postgres-meta PR | Verdict | Reasoning |
|---|---|---|
| #1082 | Ported | Real invalid-syntax bug, correct approach. |
| #1094 | Ported | Import failure on Python 3.9/3.10, independently
verified by community comments on the PR. |
| #1129 | Ported | Every JSON column failed validation at runtime;
`JsonValue` is pydantic's native type for a parsed JSON value. |
| #1063 (python part) | Reimplemented | Real bug, but the PR's Python
changes did not actually fix it (dead code); the underlying fix is one
line in `typeToClass`. |
| #1072 (`frozen=True`) | Skipped | Author-labeled feature and an
opinionated behavior change that breaks consumers who mutate row models;
belongs behind a generator option if wanted. |
| #808 | Skipped | 2023 draft fully superseded by the
maintainer-authored template this package ports. |

## Validation

- Unit tests per fix (pathological enum labels and aliases, import block
assertions, json/jsonb mapping, composite `Optional` fields).
- Parity golden regenerated (39 lines): the `typing_extensions` import
split, 16 `Json[Any]` to `JsonValue` occurrences, and two composite
attributes gaining `Optional[...]`; reviewed line by line and the golden
gate was verified to actually trip on corruption.
- The regenerated golden imports cleanly under pydantic, passes `mypy`,
and runtime checks confirm deserialized JSON and `None` composite fields
now validate.
- `check-types`, `format-and-lint`, `knip`, `build`, `test` (97 pass,
includes Docker-backed introspection and parity) all green.
- Note: the nightly parity job against real postgres-meta will show this
intentional drift until postgres-meta consumes a release containing it
(supabase/postgres-meta#1084 replaces the templates with this package,
closing the gap).