fix(jsonapi): allow opt-in client-generated IDs on POST per spec

[abderrahimghazali]

Q	A
Branch?	main
Bug fix?	yes
New feature?	no (opt-in flag)
Deprecations?	no
Issues	Closes #6738
License	MIT

What's in this PR?

Two related JSON:API issues prevent valid POST requests with client-generated IDs (per JSON:API §7.3):

1. Schema (SchemaFactory) declared data.id required for every operation, including the request body of a POST. The spec says id MAY be supplied by the client and is otherwise optional on creation.
2. Denormalizer (ItemNormalizer) treated any incoming data.id as a hint to load an existing resource, then either threw Update is not allowed for this operation or failed to resolve the IRI when the client passed a fresh UUID.

Together they make it impossible to POST {"data":{"type":"…","id":"<uuid>","attributes":{…}}} even when the application is designed for client-generated identifiers (Doctrine UUID PK, ULID, etc.).

Fix

JsonApi\JsonSchema\SchemaFactory

• For Schema::TYPE_INPUT on a Post operation, data.required is now ["type"]. Output schemas and non-Post operations remain ["type", "id"] (response payloads still always carry an id).
• The active operation is captured at the start of buildDefinitionPropertiesSchema() because the relationship loop reassigns \$operation.

Before (POST request body):
```json
"required": ["type", "id"]
```
After:
```json
"required": ["type"]
```

JsonApi\Serializer\ItemNormalizer

• New opt-in context flag ItemNormalizer::ALLOW_CLIENT_GENERATED_ID ('allow_client_generated_id').
• On a Post, an incoming data.id no longer triggers an existing-resource lookup.
  • If the flag is off (default): throws NotNormalizableValueException with a clear message — no behaviour change for endpoints that don't expect client-generated IDs, and no risk of silently letting a client spoof an ID.
  • If the flag is on: the id is merged into the denormalized payload and applied to the new entity via the property setter. The IRI converter is not queried.
• Existing PUT/PATCH path (load by IRI / OBJECT_TO_POPULATE) is preserved.

The flag is off by default to keep the change non-breaking and to avoid an ID-spoofing footgun on public endpoints. Applications opt in per-operation by passing the flag in the denormalization context (or via a state processor / serializer context builder).

Tests

• SchemaFactoryTest::testBuildSchemaForPostInputDoesNotRequireId — POST input drops id from required.
• SchemaFactoryTest::testBuildSchemaForPostOutputStillRequiresId — POST output still requires id.
• ItemNormalizerTest::testDenormalizePostWithIdThrowsWithoutOptIn — POST with client id throws without opt-in.
• ItemNormalizerTest::testDenormalizePostWithIdSucceedsWithOptIn — POST with client id succeeds with opt-in, IRI converter is not called, id is set on the new entity.

Full src/JsonApi/Tests suite: 57 tests / 145 assertions, all green (the 2 PHPUnit notices are pre-existing).

Spec references

• https://jsonapi.org/format/#crud-creating
• https://jsonapi.org/format/#crud-creating-client-ids

Credit to @cay89 for the original analysis in #6738.