Check cohort_id value against cohort_name column in registry

[ateucher]

This gives an informative error if you call call_agenda() with a cohort_id that doesn't match a cohort name in the registry.

Now I get:

registry_url <- "the-registry-url"

call_agenda(registry_url = registry_url, 
                cohort_id = "not-a-cohort", 
                call_number = 2)

#> ✔ Reading from "OpenscapesChampionsCohortRegistry".
#> ✔ Range ''cohort_metadata''.
#> New names:
#> Error in `call_agenda()`:
#> ! `cohort_id` "not-a-cohort" not found in cohort registry. Check
#>   `cohort_id` against cohort names in registry.

^{Created on 2025-08-26 with reprex v2.1.1}

Closes #117

[stefaniebutland]

Tested. Works - informatively - for me thank you!

[seankross]

lgtm