Support MantisBT REST roots in MANTISBT_URL

[shbernal]

Some MantisBT installations expose the REST API through an API root such as /api/rest/index.php instead of pretty routes under /api/rest.

mantisbt-cli currently reads MANTISBT_URL, but command routes also include the /api/rest prefix. That means MANTISBT_URL can only be a web root. For an instance whose working endpoint is:

https://example.com/api/rest/index.php/users/me

there is no value that produces the correct URL today:

MANTISBT_URL=https://example.com
-> https://example.com/api/rest/users/me

MANTISBT_URL=https://example.com/api/rest
-> https://example.com/api/rest/api/rest/users/me

MANTISBT_URL=https://example.com/api/rest/index.php
-> https://example.com/api/rest/index.php/api/rest/users/me

Expected behavior: MANTISBT_URL should support both existing web-root configuration and REST API roots, including /api/rest and /api/rest/index.php, without duplicating the /api/rest path segment.

This keeps the current documented setup working while allowing deployments that require index.php in REST URLs.