veb: fix veb silently fails to bind a handler when result type is not veb.Result

[bradcypert]

Potential Fix for #25970.

Fixes an issue where Veb would silently fail to bind a handler when the return type of a handler function is not veb.Result (and is, for example, !veb.Result). I've also added tests to cover this. I'm happy to take a different approach if needed, too. This is my first contribution to V and am still learning the ropes.

Changes:

• Added a function to check for route attributes which is used in other items below
• When generating routes, adds a branch to handle the case where a method does not have a veb.Result return type, but does have route attributes, in which case, error to warn the user of unexpected behavior.

[spytheman]

Welcome to the V repo, and thanks for your excellent PR.

I think it is a very good addition, and it can help prevent a lot of confusion (as seen on Discord and here).

[spytheman]

I played a bit with it, and discovered, that we are missing a source location information in the metadata for the method, but it would have been useful for situations like this 🤔 .

For example, I got:

V panic: method `get_task` has route attributes but invalid return type. Handler methods must return `veb.Result`, not `!veb.Result` or other types

If we had a location information, then the error message could become:

V panic: method `vvv.v:13:18:get_task` has route attributes but invalid return type. Handler methods must return `veb.Result`, not `!veb.Result` or other types

I'll see what can be done about it asap.

[spytheman]

With latest V 22fafd6, the error is now:

method `get_task` at `vlib/veb/tests/invalid_return_type_test.v:25:18` has route attributes but invalid return type. Handler methods must return `veb.Result`, not `!veb.Result` or other types