refactor: align conversation direct uploads with standard account auth (#15039)

Conversation attachment uploads now go through the same authentication
that every other account-scoped API endpoint uses. Agents continue to
attach files exactly as before, and the upload request is now tied to
the agent's dashboard session instead of a separately serialized access
token.

Because the upload request is now authenticated, the dashboard proves
the agent's session directly instead of passing
`currentUser.access_token`. This keeps uploads working alongside the
profile access-token changes in #14973, including on accounts where that
token is serialized as empty.

## What changed

- `Api::V1::Accounts::Conversations::DirectUploadsController` now runs
the standard account auth stack: API access token when the
`api_access_token` header is present, dashboard session
(devise-token-auth) otherwise, with agent-bot tokens rejected.
Previously it inherited `ActiveStorage::DirectUploadsController`
directly and did not run any authentication.
- `EnsureCurrentAccountHelper#ensure_current_account` now returns `401`
when a request has neither an authenticated user nor a bot resource,
instead of continuing. This closes the same gap for any controller that
relies on the helper.
- The dashboard direct-upload paths (`useFileUpload.js` and the legacy
`fileUploadMixin.js`) now attach the agent's session headers to the
upload request via a new `directUploadsHelper.js`, instead of sending
`currentUser.access_token`.

## How to test

1. As a logged-in agent, open a conversation and attach a file. Upload
should succeed as before, on installs with direct uploads enabled.
2. Confirm attachments still work for an agent on an account whose
profile access token is not serialized (e.g. a Cloud plan without
`api_and_webhooks`).
3. Send a `POST` to
`/api/v1/accounts/:account_id/conversations/:conversation_id/direct_uploads`
with no credentials, an empty `api_access_token`, or an invalid token,
and confirm it returns `401`.
4. Confirm a valid agent of the account (via API token or session) gets
`200`, while an agent of a different account gets `401`.

app/javascript/dashboard/mixins/fileUploadMixin.js

@@ -3,6 +3,7 @@ import { useAlert } from 'dashboard/composables';
 import { checkFileSizeLimit } from 'shared/helpers/FileHelper';
 import { getMaxUploadSizeByChannel } from '@chatwoot/utils';
 import { DirectUpload } from 'activestorage';
+import { setDirectUploadAuthHeaders } from 'dashboard/helper/directUploadsHelper';
 import {
   DEFAULT_MAXIMUM_FILE_UPLOAD_SIZE,
   resolveMaximumFileUploadSize,
@@ -77,10 +78,7 @@ export default {
         `/api/v1/accounts/${this.accountId}/conversations/${this.currentChat.id}/direct_uploads`,
         {
           directUploadWillCreateBlobWithXHR: xhr => {
-            xhr.setRequestHeader(
-              'api_access_token',
-              this.currentUser.access_token
-            );
+            setDirectUploadAuthHeaders(xhr);
           },
         }
       );