fix(agent-bots): destroy permissibles on AgentBot deletion and skip orphans in index (#14273)

\`GET /platform/api/v1/agent_bots\` returns 500 when any \`AgentBot\`
that was previously registered with a Platform App has since been
deleted. The bug was introduced by a missing \`dependent: :destroy\` on
the \`AgentBot\` model — deleting a bot left orphaned rows in
\`platform_app_permissibles\`, which the index action later iterated
over and crashed rendering with a \`NoMethodError\` on \`nil\`.

Closes #13407

## Root cause

The index action loads all \`platform_app_permissibles\` for the
platform app and passes each \`resource.permissible\` (the associated
\`AgentBot\`) to a Jbuilder partial. When the \`AgentBot\` no longer
exists, \`resource.permissible\` returns \`nil\` and the partial crashes
calling \`.id\`, \`.name\`, etc. on it.

Every other \`AgentBot\` association (\`agent_bot_inboxes\`,
\`messages\`, \`assigned_conversations\`) had a \`dependent:\` option —
\`platform_app_permissibles\` was the only one missing it. There was
also an N+1 query: the index fired a separate SQL query per permissible
to load each bot.

## What changed

**1. Model — prevent orphans at deletion time**
\`\`\`ruby
has_many :platform_app_permissibles, as: :permissible, dependent:
:destroy
\`\`\`

**2. Controller — eager-load to eliminate N+1**
\`\`\`ruby
@resources = @platform_app.platform_app_permissibles
               .where(permissible_type: 'AgentBot')
               .includes(:permissible)
\`\`\`

**3. Jbuilder — defensive nil guard for pre-existing orphans**
\`\`\`ruby
bot = resource.permissible
next if bot.nil?
json.partial! '...', resource: bot
\`\`\`

## Trade-offs considered

| Option | Decision |
|---|---|
| Rescue \`NoMethodError\` in jbuilder | Hides the failure rather than
fixing it. Rejected. |
| Only add the nil guard, skip the model fix | Leaves the data integrity
gap open — future deletions continue creating orphans. Rejected. |
| Both layers (chosen) | Model fix prevents new orphans; nil guard is
defence-in-depth for any orphans that survived before deployment. |
| \`dependent: :nullify\` | Doesn't apply — a nullified permissible
would still cause the same nil dereference. Rejected. |

## How to reproduce

1. Create an AgentBot via the Platform API
2. Delete the AgentBot via any path (admin UI, API, or direct model
call)
3. Call \`GET /platform/api/v1/agent_bots\` with a Platform App token
4. Observe 500

After this fix, the endpoint returns 200 with an empty array.

Co-authored-by: Ramalau Debeila <rdebeila@datacentrix.co.za>

app/controllers/platform/api/v1/agent_bots_controller.rb

@@ -3,7 +3,7 @@ class Platform::Api::V1::AgentBotsController < PlatformController
   before_action :validate_platform_app_permissible, except: [:index, :create]
 
   def index
-    @resources = @platform_app.platform_app_permissibles.where(permissible_type: 'AgentBot').all
+    @resources = @platform_app.platform_app_permissibles.where(permissible_type: 'AgentBot').includes(:permissible)
   end
 
   def show; end