Convert nested-intersection-union unit test to a schema fixture test

[Mark Probst (schani)]

What this does

Converts the Vitest unit test from glideapps#2914 (test/unit/nested-intersection-union.test.ts) into a standard JSON Schema fixture test:

• Adds test/inputs/schema/nested-intersection-union.schema — a trimmed repro of FlattenUnions fails when unifying an array element union that indirectly contains an intersection. glideapps/quicktype#2913: an intersection (allOf) nested inside a union (oneOf), reached through an array, which crashed FlattenUnions with "Internal error: Unknown type intersection" before the fix on this branch.
• Adds nested-intersection-union.1.json — positive instance covering all union variants (fully-populated item plus Message-only, FunctionOutput-only, and ReasoningItem-only partials), round-tripped by the harness.
• Adds nested-intersection-union.1.fail.union.json — negative example (structural mismatch: array where an item object is expected), following the class-map-union fixture precedent.
• Removes the unit test — it only asserted three substrings in Rust output; the fixture strictly supersedes it: it exercises the fix across every language in CI, with real round-trip verification and a negative case.

Why

quicktype's maintainers prefer regression coverage for type-graph fixes as schema fixtures rather than renderer-output string assertions — fixtures run for all 22 CI target languages automatically, so the fix is exercised end-to-end everywhere, not just in Rust.

Verification

• Full monorepo build; unit test suite passes after the removal (59/59).
• All 22 CI target languages generate successfully from the new schema (no skipSchema entries needed).
• End-to-end fixture harness pass locally for schema-golang and schema-typescript (positive round-trip + expected failure both run); remaining compiled-language toolchains are covered by CI.

Merging this into intersection-fix will update glideapps#2914 automatically.

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