Lacking of capabilityManager should not raise error

Description

When working on self hosting, i found that when generating oauth token, the endpoint will return error about planIdsToClientCapabilities

POST /v1/oauth/token HTTP/1.1
Host: api.fxa.example.local
authorization: Bearer fxs_xxxxx

which returns 500
{"code":500,"errno":998,"error":"Internal Server Error","message":"An internal validation check failed.","info":"https://mozilla.github.io/ecosystem-platform/api#section/Response-format","op":"planIdsToClientCapabilities","data":{}}

some digging and background:

1. For selfhosting, capabilityManager is not instantiated.

Since these requirements config.subscriptions && config.subscriptions.stripeApiKey and config.cms.enabled || (config.cms.strapiClient && config.cms.strapiClient.graphqlApiUri && config.cms.strapiClient.apiKey && config.cms.strapiClient.firestoreCacheCollectionName in key_server.js prevents capabilityManager being instantiated.

2. planIdsToClientCapabilities is not skipable too.

According to scope docs , when Firefox browser send scope profile which will definitely hit capabilityService.subscriptionCapabilities in oauth/grant.js:generateAccessToken since profile:subscriptions is a sub-scope of profile
and planIdsToClientCapabilities will be called, and then error happens.

So there's nothing self-hosting side can do (except patching these code).

What should happen?

/v1/oauth/token should return token whether capabilityManager is available or not

Actual result

500 Internal Server Error

Environment

Firefox 154.0

┆Issue is synchronized with this Jira Task

Start with packages/fxa-auth-server/bin/key_server.js to understand when capabilityManager is instantiated, then trace generateAccessToken in packages/fxa-auth-server/lib/oauth/grant.js through subscriptionCapabilities and planIdsToClientCapabilities in packages/fxa-auth-server/lib/payments/capability.ts. Reproduce POST /v1/oauth/token in a self-hosted configuration without capabilityManager; done means it returns a token instead of the 500 validation error.