Fix agents disabled and disabledReason fields in APIs

What does this MR do and why?

The logic for checking whether a composite identity (agent) user is disabled was duplicated across three places: EE::Types::UserStatusType, Types::Users::DuoStatusType, and EE::Users::ParticipableService.

Also, the logic was incorrect in all 3 types and instead of checking credits of current_user, the actual user/agent was passed on to UsageQuotaService, making it to fail because service accounts won't have credits assigned to them.

This MR:

• Centralizes the agent?(user) guard (service_account? && composite_identity_enforced?)
• Maps all UsageQuotaService error reasons to human-readable messages
• Fixes the quota usage check to use the correct user (current_user) instead of the user whose availability is being checked

This is a refactor with no functional change for the "no credits" path. The additional failure reasons (:user_missing, :namespace_missing, unknown) were previously swallowed and returned no message — they now surface a proper reason string, which will help debug issues like the one reported in #593981 (closed) where the duo developer service account was not showing up in the mention autocomplete.

References

Closes #593981 (closed)

How to set up and validate locally

No functional change for the happy path. To validate the disabled state:

1. Apply the patch below to force credits to be exhausted:

diff --git a/ee/app/services/ai/usage_quota_service.rb b/ee/app/services/ai/usage_quota_service.rb
--- a/ee/app/services/ai/usage_quota_service.rb
+++ b/ee/app/services/ai/usage_quota_service.rb
@@ -36,7 +36,7 @@ def execute
-      if usage_quota_response(params.compact).dig("data", "errors")&.include?("402")
+      if true || usage_quota_response(params.compact).dig("data", "errors")&.include?("402")

2. Make sure you have at least one foundational flow enabled for a group with Duo enabled
3. Go to an MR or issue under that group and try to @-mention or assign the service account — it should show as disabled with the correct reason message

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