Improve performance by using global namespace prefix for all functions

[clue]

This changeset improves performance slightly in synthetic benchmarks by using the global namespace prefix \ for all global functions. This was already in place for most invocations, now it is used consistently throughout the entire code base.

Refs reactphp/stream#137 and others

[Copilot]

Pull request overview

This PR improves performance by consistently using the global namespace prefix \ for all global PHP function calls throughout the codebase. This optimization was already partially in place, and now it's applied uniformly across all files.

• Adds global namespace prefix \ to all remaining global function calls
• Maintains consistency with existing code that already used this optimization
• Applies changes across 9 files covering various components (SAPI handler, routing, middleware, logging, container, etc.)

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Io/SapiHandler.php	Added \ prefix to string manipulation, I/O, and PHP configuration functions
src/Io/RouteHandler.php	Added \ prefix to array functions and assertions
src/Io/MiddlewareHandler.php	Added \ prefix to assert and count functions
src/Io/LogStreamHandler.php	Added \ prefix to assert function
src/Io/FiberHandler.php	Added \ prefix to assert function
src/FilesystemHandler.php	Added \ prefix to assert function
src/Container.php	Added \ prefix to type checking, assertion, and class existence functions
src/App.php	Added \ prefix to assert function
src/AccessLogHandler.php	Added \ prefix to string, time, and encoding functions

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