Use floor() instead of round() in rest_is_integer()

[siliconforks]

This fixes a problem with large integers causing rest_is_integer() to return false.

It also adds some tests for large integers and a few other things that did not have any existing tests (negative floats, scientific notation).

To run the test suite:

vendor/bin/phpunit --filter Tests_REST_API

Trac ticket: https://core.trac.wordpress.org/ticket/65271

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Opus 4.7
Used for: @westonruter said that Claude Opus 4.7 made suggestions to add separate branches for int values and string integers; these looked fine to me, so I added them.

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Props siliconforks, westonruter.

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