fix(openapi): emit valid 3.0 schemas when downgrading from 3.1

[soyuka]

Summary

LegacyOpenApiNormalizer (triggered by ?spec_version=3.0.0 and api:openapi:export --spec-version=3.0.0) produced invalid OpenAPI 3.0 output because it only ran a shallow scan over components.schemas.*.properties.*. Problems with the previous code:

• type: ['array', 'null'] + items → anyOf: [{type:array},{type:null}] with an orphan items sibling (invalid in 3.0).
• type: 'null' entries leaked through — null is not a valid type in OpenAPI 3.0.
• No recursion into nested embeddables (properties.properties), items, additionalProperties, allOf/oneOf/anyOf.
• Path-level inline schemas (requestBody.content.*.schema, responses.*.content.*.schema, parameters.*.schema) were never visited.
• examples → example conversion was also shallow.

Fix

Rewrite as a recursive schema walk:

• Nullable types map to type: X, nullable: true (3.0 idiomatic), preserving items / properties / additionalProperties.
• Multi non-null type unions fall back to anyOf + nullable: true.
• Recursion through properties, patternProperties, items, additionalProperties, allOf, oneOf, anyOf, not, contains, propertyNames, if/then/else.
• Walks paths.*.{method}.requestBody.content.*.schema, responses.*.content.*.schema, parameters.*.schema.
• examples → example applied recursively.

Test plan

• Added src/OpenApi/Tests/Serializer/LegacyOpenApiNormalizerTest.php (10 cases) covering: untouched 3.1 default, nullable scalar, nullable array with items, nested embeddable, items recursion, allOf/oneOf/anyOf, additionalProperties, multi non-null union → anyOf, recursive examples→example, path operation request body walk.
• vendor/bin/phpunit src/OpenApi/Tests/Serializer/LegacyOpenApiNormalizerTest.php — 10/10 pass.
• vendor/bin/phpunit src/Symfony/Tests/Action/DocumentationActionTest.php — 11/11 pass.
• Remaining OpenApi suite failure (OpenApiFactoryTest::testInvoke description text) is pre-existing on 4.3, unrelated to this change.

Refs #6194 — addresses the embeddable nullable array case reported there for users who use the 3.0 downgrade as a workaround. The root issue (zircote/swagger-php not supporting OpenAPI 3.1) remains upstream.