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/contact_conversation_message.yml

@@ -31,9 +31,10 @@ properties:
     type: string
     description: Status of the message
   source_id:
-    type: string
+    type:
+      - string
+      - 'null'
     description: Source ID of the message
-    nullable: true
   content_type:
     type: string
     description: Type of the content
@@ -41,13 +42,15 @@ properties:
     type: object
     description: Attributes of the content
   sender_type:
-    type: string
+    type:
+      - string
+      - 'null'
     description: Type of the sender
-    nullable: true
   sender_id:
-    type: integer
+    type:
+      - integer
+      - 'null'
     description: ID of the sender
-    nullable: true
   external_source_ids:
     type: object
     description: External source IDs
@@ -55,9 +58,10 @@ properties:
     type: object
     description: Additional attributes of the message
   processed_message_content:
-    type: string
+    type:
+      - string
+      - 'null'
     description: Processed message content
-    nullable: true
   sentiment:
     type: object
     description: Sentiment analysis of the message
@@ -66,9 +70,10 @@ properties:
     description: Conversation details
     properties:
       assignee_id:
-        type: integer
+        type:
+          - integer
+          - 'null'
         description: ID of the assignee
-        nullable: true
       unread_count:
         type: integer
         description: Count of unread messages