fix(analysis): requiredness branches may restate type: object

[iamralch]

Fixes #88.

What was wrong

A union whose branches only list required properties constrains the object; it isn't a variant of it. union_only_constrains_requiredness already treats it that way, but only when each branch has nothing but required, not, nested unions and annotations. A branch that also says type: object was read as an empty object variant:

Email:
  type: object
  required: [from]
  properties: { from: ..., to: ..., cc: ..., text: ..., html: ... }
  allOf:
    - anyOf:
        - { type: object, required: [to] }
        - { type: object, required: [cc] }
    - anyOf:
        - { type: object, required: [text] }
        - { type: object, required: [html] }

With two of them in an allOf, generation failed:

Error: Invalid schema: allOf object `Email` intersects multiple union members (`EmailAllOfVariant1` and `EmailAllOfVariant2`), which cannot be represented by one flattened variant

and one beside a real union competed with it for the variant.

The change

schema_only_constrains_requiredness also accepts type: "object" in a branch, and nothing else for type. Only an object has required properties, so it says nothing the requiredness doesn't.

Tests

In tests/recoverable_typing_test.rs, beside the existing requiredness case:

• requiredness_branches_that_restate_type_object_are_still_only_requiredness: the schema above, Cloudflare's Email Sending builder, is a struct of its properties.
• a_requiredness_union_beside_a_real_union_leaves_that_union_the_variant: Cloudflare's Magic WAN app config, a oneOf of account or managed app beside an anyOf of required: [breakout] or required: [priority], keeps the oneOf as its variant.

Both fail before this change. cargo test --all-features: 720 passed, none failed.

The corpus doesn't move: scripts/gen-diff.sh upstream/main reports every spec identical, since no pinned spec has such a branch. The pinned cloudflare spec predates both of these schemas. Against the current cloudflare/api-schemas, both now generate, without the overlay we used to work around them.

#88 lists a third Cloudflare case, the Workers Observability filters: an allOf of a $ref to a union and an inline copy of the same union. That isn't requiredness, and this doesn't change it.

This and #90 both add a Fixed entry under [Unreleased] in CHANGELOG.md, so whichever merges second needs that resolved. I'll rebase.

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