fix: return correct outgoing_url in Platform agent bot API responses (#13827)

The Platform API was returning the bot's `name` value for the
`outgoing_url` field across all agent bot endpoints (index, show,
create, update). A typo in the `_agent_bot.json.jbuilder` partial used
`resource.name` instead of `resource.outgoing_url`.

Closes #13787

## What changed

- `app/views/platform/api/v1/models/_agent_bot.json.jbuilder`: corrected
`resource.name` → `resource.outgoing_url` on the `outgoing_url` field.

## How to reproduce

1. Create an agent bot via `POST /platform/api/v1/agent_bots` with a
distinct `outgoing_url`.
2. Call `GET /platform/api/v1/agent_bots/:id`.
3. Before this fix the `outgoing_url` in the response equals the bot's
`name`; after the fix it equals the value set at creation.

## Tests

Added `outgoing_url` assertions to the existing GET index and GET show
request specs so the regression is covered.

app/views/platform/api/v1/models/_agent_bot.json.jbuilder

@@ -1,6 +1,6 @@
 json.id resource.id
 json.name resource.name
 json.description resource.description
-json.outgoing_url resource.name
+json.outgoing_url resource.outgoing_url
 json.account_id resource.account_id
 json.access_token resource.access_token.token