Throw error if required route params missing

[askvortsov1]

Fixes #2513

Changes proposed in this pull request:

• Throw an explicit exception if a required path argument is missing when generating URls
• Restructure fixPathPart method to be clearer
• Call fixPathPart functionally via array_map` instead of using pass-by-reference to update the part magically.
• Add tests to ensure proper behavior for required and optional parameters.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.