Fix registering custom searchers, allow searchers without fulltext

[askvortsov1]

Fixes #2712

Changes proposed in this pull request:

• Fix registering custom searchers
• Add integration tests
• Require FulltextGambit as constructor arg for GambitManager

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[SychO9]

if there doesn't exist a fulltext gambit for a model, then it's not really "searchable", and as such, a filterer should be used.

I agree, extension developers might try to use gambits when what they really need is filters instead. Throwing an exception somewhere -if we can- that makes it clear would be great as well.

[clarkwinkelmann]

I have not tested locally, but this looks good to me.

I was worried that gambits might be resolved too early, but that doesn't seem to be an issue. The array is resolved in boot, but the actual classes are only container->make() inside of the container binding so it's fine.

My only issue is the comment below.

[askvortsov1]

Reverted the exception as I was unable to catch it.

[SychO9]

Had a similar problem with trying to catch an exception from tests, it just wasn't possible.