JSON Schemas: Add SCF_Schema_Builder for field schema composition

[priethor]

What

Required for #162.

Adds a SCF_Schema_Builder class to handle JSON Schema composition for field type validation.

Why

WordPress internal JSON validation uses V4 and doesn't understand JSON Schema $ref or complex schema composition. Field validation requires type-specific rules while sharing:

• Base properties common to all 35 field types.
• Properties shared by all/most fields in the same category that could be defined once and referenced, instead of defined multiple times.

This builder enables strict per-type validation through oneOf composition and extracts the existing $ref resolution.

How

Extracting schema building logic from abilities classes into a dedicated SCF_Schema_Builder class that:

• Resolves $ref references by inlining definitions for WordPress compatibility
• Composes field schemas with oneOf variants for each field type

Since WordPress validation doesn't support allOf for the field composition, what we do here for fields at runtime is:

1. Load the base properties common to all fields
2. Load each partial field schema that has its specific fields
3. Resolve any references for reusable properties that are not common to all fields but defined once
4. Compose a full JSON schema for the field, with all properties.
5. The validation will accept oneOf each complete field schema.

This PR also adds the most basic field schema, text.schema.json, as the first type-specific schema to validate the composition pipeline.

Testing Instructions

1. Run vendor/bin/phpunit tests/php/includes/SCFSchemaBuilderTest.php - all 8 tests should pass
2. Run full test suite vendor/bin/phpunit - all 404 tests should pass
3. Run composer test:phpstan - no errors

[codecov]

Codecov Report

❌ Patch coverage is 95.14563% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.46%. Comparing base (8887e6c) to head (f35baf7).

Files with missing lines	Patch %	Lines
includes/class-scf-schema-builder.php	94.11%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #301      +/-   ##
============================================
+ Coverage     14.36%   14.46%   +0.09%     
- Complexity     4386     4397      +11     
============================================
  Files           286      287       +1     
  Lines         35270    35320      +50     
============================================
+ Hits           5066     5108      +42     
- Misses        30204    30212       +8     

Flag	Coverage Δ
e2e	30.69% <ø> (-0.03%)	⬇️
javascript	2.60% <ø> (ø)
phpunit	10.61% <95.14%> (+0.14%)	⬆️

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[github-actions]

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Props priethor.

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