[qorud02]

The REST API client's setMeta() and setMetas() mutate the model's existing metadata object before passing it to Backbone's set(). The comparison then sees the same object and does not emit change:meta. When metadata is missing, the single setter throws and the plural setter does not retain the supplied values.

Copy the existing metadata into a new object before updating it. This preserves other keys and the previous model value, emits an event when metadata changes, and keeps unchanged values silent.

Add QUnit coverage for both setters: initializing missing metadata, updating existing metadata, preserving other keys and previous values, and suppressing redundant events.

Trac ticket: https://core.trac.wordpress.org/ticket/43149

Testing:

• The isolated regressions execute the actual Core mixin and test block: the baseline fails both tests; the patch passes both tests (16 assertions).
• The complete wp-api.js QUnit test file, with Core's real schema and AJAX fixtures, passes 152/152 assertions through the unchanged official Playwright QUnit runner. A focused HTML page loads these assets; this is the API client suite, not the entire Core JavaScript suite.
• node --check for both changed JavaScript files and git diff --check pass.
• Runtime: Node 24.21.0, npm 11.19.0, Playwright 1.61.1, Backbone 1.6.1, Underscore 1.13.8.

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Used for: Repository investigation, patch and regression-test preparation, and automated local verification. Human maintainer review is requested before acceptance.

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This pull request is for code review only. Please keep other discussion in the Trac ticket. Do not merge this pull request.

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