feat: validate OpenAPI spec using Skooma (#13623)

Adds Skooma-based OpenAPI validation so SDK-facing request specs can
assert that documented request and response contracts match real Rails
behavior. This also upgrades the spec to OpenAPI 3.1 and fixes contract
drift uncovered while validating core application and platform
resources.

Closes
None

Why
We want CI to catch OpenAPI drift before it reaches SDK consumers. While
wiring validation in, this PR surfaced several mismatches between the
documented contract and what the Rails endpoints actually accept or
return.

What this change does
- Adds Skooma-backed OpenAPI validation to the request spec flow and a
dedicated OpenAPI validation spec.
- Migrates nullable schema definitions to OpenAPI 3.1-compatible unions.
- Updates core SDK-facing schemas and payloads across accounts,
contacts, conversations, inboxes, messages, teams, reporting events, and
platform account resources.
- Documents concrete runtime cases that were previously missing or
inaccurate, including nested `profile` update payloads, multipart avatar
uploads, required profile update bodies, nullable inbox feature flags,
and message sender types that include both `Captain::Assistant` and
senderless activity-style messages.
- Regenerates the committed Swagger JSON and tag-group artifacts used by
CI sync checks.

Validation
- `bundle exec rake swagger:build`
- `bundle exec rspec spec/swagger/openapi_spec.rb`

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

swagger/definitions/resource/conversation.yml

@@ -43,7 +43,9 @@ properties:
     type: boolean
     description: Whether the conversation is muted
   snoozed_until:
-    type: number
+    type:
+      - number
+      - 'null'
     description: The time at which the conversation will be unmuted
   status:
     type: string
@@ -56,29 +58,38 @@ properties:
     type: number
     description: The time at which conversation was updated
   timestamp:
-    type: string
+    type: number
     description: The time at which conversation was created
   first_reply_created_at:
-    type: number
+    type:
+      - number
+      - 'null'
     description: The time at which the first reply was created
   unread_count:
     type: number
     description: The number of unread messages
   last_non_activity_message:
-    type: object
-    $ref: '#/components/schemas/message'
+    oneOf:
+      - $ref: '#/components/schemas/message'
+      - type: 'null'
     description: The last non activity message
   last_activity_at:
     type: number
     description: The last activity at of the conversation
   priority:
-    type: string
+    type:
+      - string
+      - 'null'
     description: The priority of the conversation
   waiting_since:
-    type: number
+    type:
+      - number
+      - 'null'
     description: The time at which the conversation was waiting
   sla_policy_id:
-    type: number
+    type:
+      - number
+      - 'null'
     description: The ID of the SLA policy
   applied_sla:
     type: object