Harden allOf merging and default-response handling

[cubahno]

Summary

A grab-bag of schema-composition fixes uncovered while generating against real-world specs.
The unifying theme: keep the generated types aligned with what strict response validators actually check, including in malformed but real corners.

• Flattens allOf wrappers before the type-presence checks in mergeOpenapiSchemas. A nested allOf wrapper without its own type was being dropped by the len(t1) == 0 short-circuit, losing its properties.

• Lets a parent type: object override a primitive allOf result (type: object, allOf: [SomeEnum]) - the outer object wins so the validator sees an object.

• Merges colliding properties via mergePropertyProxies so enum / pattern / format constraints survive instead of being clobbered by the second declaration.

• Merges array items from both branches (previously dropped, with downstream inference falling back to string).

• Treats readOnly / writeOnly as annotations (OR-merge), not constraints that have to match per branch.

• isDiscriminatedUnionWithChild: checks anyOf too, and return false when a discriminator+mapping describes an inheritance base rather than a union (no oneOf / anyOf).

• When no explicit success is documented but default has content, treats default as 200 success. Fabricating a 204 struct{} made return content that didn't satisfy the real schema.

• Preserves the declared ContentType on empty success bodies (e.g. text/html: {}) - strict validators rely on it.

• Refactored shared logic into buildDefaultResponseDefinition.