fix: prevent assert string evaluation in REST controller on PHP 7

[imantsk]

Summary

On PHP 7.x with assertions active, assert() evaluates a string first argument as PHP code. REST_Controller::__construct() passed static::BASE_ROUTE (a route string) directly to assert(), so PHP tried to execute each route value as code.

Reported from a live PHP 7.4 site: every registered REST controller emitted warnings — Use of undefined constant recently/active/import/plugins/file/upload, A non-numeric value encountered (recently-active parsed as subtraction), and Division by zero (routes containing /).

PHP 8 removed string evaluation in assert(), so this only affects PHP 7.x.

Change

assert( static::BASE_ROUTE !== '', get_class( $this ) . '::BASE_ROUTE must be set' );

A boolean first argument is never evaluated as a string, on any PHP version, and preserves the original intent. The sibling Model::__construct() assert is unaffected — its argument is an array, not a string.

Verification

• phpcs clean, php -l clean.
• REST test suite: 47/47 pass.