fix(codegen/python): emit nullable Option<T> fields as optional in flattened tagged-enum models

[hardbyte]

Fixes #167

Root cause

The bug is one step removed from the issue's minimal repro: a standalone internally-tagged enum already generates correctly, because that render path routes field optionality through resolve_field_optionality (which treats any Option<T> field as an optional Python key, since serde omits the key for None).

The failing shape is a struct with a #[serde(flatten)]-ed internally-tagged enum — e.g. OffersSearchOffer flattening an offer-details enum, producing per-variant models like OffersSearchOfferProvisional. Those models are generated by render_struct_with_flattened_internal_enum / render_struct_with_flatten_standard, which hand-rolled optionality from field.required alone and ignored the Option<T> special case. Two symptoms:

• Option<T> without skip_serializing_if → amount: str | None required, no default → pydantic.ValidationError: Field required on a valid payload that omits the key
• Option<T> with skip_serializing_if → doubled union note: str | None | None = None

@MattGson note: your minimal repro only fails when the enum is flattened into a parent struct — worth sanity-checking that matches the offers code shape.

Fix

Route all five inline field-emission sites in reflectapi/src/codegen/python.rs through resolve_field_optionality:

• parent-struct base fields and flattened-sibling-enum fields in render_struct_with_flattened_internal_enum
• tuple-variant inner-struct fields in the same function
• regular fields in render_struct_with_flatten_standard
• flattened-enum-as-field emission (had its own double-| None bug for Option<Enum>)

Field optionality is now decided in exactly one place, so flattened-enum variant models match standalone struct output.

Tests

• New snapshot samples for both shapes: the issue's standalone repro (guards the already-correct path) and the flattened variant that reproduces the bug. Both run through assert_snapshot!, covering schema JSON, TypeScript, Rust, OpenAPI, and Python.
• Per @avkonst's suggestion, python codegen is now covered by the same test samples as the other languages: it was already in assert_input_snapshot!/assert_output_snapshot!/assert_snapshot!; the one gap was assert_builder_snapshot! (the 4 namespace samples), which now snapshots python too (appended last so existing snapshot indices don't churn).
• 5 existing generic_flatten_* python snapshots updated — each is the same one-line correction (if_else: X | None gaining = None); those committed snapshots had been silently capturing the bug.
• Runtime-validated the generated client under pydantic with the local reflectapi-python-runtime: model_validate({"type": "variant", ...}) with omitted optional keys now succeeds on variant models directly and through the discriminated-union root, and populated keys still parse.
• Full demo suite passes (193/193); fmt/clippy/build clean.

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Updated automatically from commit c02e9fa