Tests: Optimize REST server setup in Tests_REST_API

[SainathPoojary]

Reduces unnecessary REST server setup in Tests_REST_API.

Previously, every test created a Spy_REST_Server and initialized all REST routes, including helper tests that did not need them.

This change initializes the server only where needed using rest_get_server(). The test bootstrap continues to select Spy_REST_Server. Setup and teardown clear the server to preserve test isolation.

Performance

Measured locally across three runs:

Run	Trunk Test Time	Trunk Wall Time	Fix Test Time	Fix Wall Time
Run 1	1.593s	1.884s	0.558s	0.852s
Run 2	1.756s	2.049s	0.535s	0.823s
Run 3	1.578s	1.877s	0.551s	0.850s
Average	1.642s	1.937s	0.548s	0.842s

Trac ticket: https://core.trac.wordpress.org/ticket/66074

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Used for: Edge-case checks, and assistance with the PR description. Implementation and testing were made and reviewed by me.

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Props sainathpoojary, mindctrl.

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[mindctrl]

I don't think these are needed, since it's just nulling out the server anyway. The test passes without these lines, since it's just testing rest_preload_api_request().

Suggested change

	rest_get_server();
	$rest_server = $GLOBALS['wp_rest_server'];
	$GLOBALS['wp_rest_server'] = null;

[mindctrl]

Suggested change

The $GLOBALS['wp_rest_server'] = $rest_server; is not needed anymore

[mindctrl]

Suggested change

	rest_get_server();
	$rest_server = $GLOBALS['wp_rest_server'];
	$GLOBALS['wp_rest_server'] = null;