Fix PORO resource creation without find_by_key override

[takaokouji]

Problem

When creating a PORO (Plain Old Ruby Object) resource that:

• Has a model class without the .all method
• Does NOT override find_by_key
• Does NOT override find_fragments

The create operation would fail with NoMethodError: undefined method 'all'.

Root Cause

The current implementation of Processor#create_resource calls find_resource_set after creating a resource, which internally calls:

1. find_fragments
2. find_fragments_for_non_active_record (for PORO)
3. find_by_key (default implementation)
4. records_base
5. _model_class.all ❌ NoMethodError

Solution

This PR restores the v0.9.x behavior for PORO models: when _model_class doesn't respond to :all, the created resource is passed directly to ResourceSet without attempting to re-fetch it.

ActiveRecord resources continue to use the optimized find_resource_set path with efficient queries and includes.

Changes

• Modified lib/jsonapi/processor.rb - Added PORO detection in create_resource method
• Added Test coverage for PORO resources without find_by_key override

Testing

✅ All existing tests pass:

• Rails 6.1.7.10: 705 runs, 8836 assertions, 0 failures
• Rails 7.0.10: 705 runs, 8836 assertions, 0 failures
• Rails 7.1.6: 705 runs, 8842 assertions, 0 failures

✅ New test added: test_create_poro_without_find_by_key_override

Backward Compatibility

This change maintains full backward compatibility:

• ActiveRecord resources: No behavior change (uses optimized path)
• PORO with find_by_key override: No behavior change
• PORO without overrides: Now works correctly (previously failed)

🤖 Generated with Claude Code