Fix #73630: Built-in Weberver - overwrite $_SERVER['request_uri']

The built-in Webserver's `on_path`, `on_query_string` and `on_url`
callbacks may be called multiple times from the parse

Fix #73630: Built-in Weberver - overwrite $_SERVER['request_uri']

The built-in Webserver's `on_path`, `on_query_string` and `on_url`
callbacks may be called multiple times from the parser; we must not
simply replace the old values, but need to concatenate the new values
instead.

This appears to be tricky for `on_path` due to the path normalization,
so we fail if the function is called again.

The built-in Webserver logs errors during request parsing to stderr,
but this is ignored by the php_cli_server framework, and apparently the
Webserver does not send a resonse at all in such cases (instead of an
4xx). Thus we can only check that a request with an overly long path
fails.

Closes GH-7207.

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