REST API: Allow null template origin

[sunilwordpress]

Description

The REST API schema for the origin property of the wp/v2/template-parts endpoint currently declares the value as a string.

However, WP_Block_Template::origin can also return null, so the schema does not accurately describe the actual REST API response.

This change updates the origin schema to accept both string and null values.

Trac ticket: https://core.trac.wordpress.org/ticket/64168

Testing

• PHPUnit tests for WP_REST_Templates_Controller pass.
• Result: 49 tests, 143 assertions.

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT
Model(s): GPT-5.6 Luna
Used for: Reviewing the proposed change, discussing the implementation and test approach, and troubleshooting the Git/GitHub contribution workflow. The final code change was reviewed and tested by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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