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/message_detailed.yml

@@ -18,7 +18,7 @@ properties:
     description: "The type of the message (0: incoming, 1: outgoing, 2: activity, 3: template)"
   content_type:
     type: string
-    enum: ["text", "input_select", "cards", "form", "input_csat"]
+    enum: ["text", "input_text", "input_textarea", "input_email", "input_select", "cards", "form", "article", "incoming_email", "input_csat", "integrations", "sticker", "voice_call"]
     description: The type of the message content
   status:
     type: string
@@ -29,13 +29,15 @@ properties:
     description: The content attributes for each content_type
     properties:
       in_reply_to:
-        type: string
+        type:
+          - string
+          - 'null'
         description: ID of the message this is replying to
-        nullable: true
   echo_id:
-    type: string
+    type:
+      - string
+      - 'null'
     description: The echo ID of the message, used for deduplication
-    nullable: true
   created_at:
     type: integer
     description: The timestamp when message was created
@@ -43,9 +45,10 @@ properties:
     type: boolean
     description: The flag which shows whether the message is private or not
   source_id:
-    type: string
+    type:
+      - string
+      - 'null'
     description: The source ID of the message
-    nullable: true
   sender:
     $ref: '#/components/schemas/contact_detail'
     description: The sender of the message (only for incoming messages)
@@ -76,4 +79,4 @@ properties:
           description: The thumbnail URL of the attached file
         file_size:
           type: number
-          description: The size of the attached file in bytes
+          description: The size of the attached file in bytes