[Bug]: Outbound Subscription fetch fails with HTTP 404 due to missing X-HWID (related to #6559)

Describe the bug

Hello!

Note: The logic for this feature was already successfully implemented for client links in: [Feature]: External subscription fetch sends no X-HWID, so an HWID-limited 3x-ui provider returns 404 #6559 (PR #6567).

I tested the latest dev build and the External Subscriptions sync now works perfectly. Thank you! However, I noticed that the exact same problem still exists in a different part of the panel, specifically in: Outbounds -> Subscriptions -> Outbound Subscriptions

Description: When I add an HWID-limited subscription link from a Donor node into the Master node's "Outbound Subscriptions" menu, it fails to connect and throws the following error: Failed to fetch/parse subscription (http 404)

Just like in the previous issue, the donor's HWID gate blocks the request because the internal fetcher for Outbound Subscriptions also does not send the X-HWID header.

Suggested Solution: Could the exact same logic (GetPanelGuid() + X-Device-OS: 3x-ui) implemented in #6559 for external_subscription.go be applied to the fetcher in outbound_subscription.go as well?

Thank you!

How to reproduce the problem

1. Set up Node 1 (Master) and Node 2 (Donor).
2. On Node 2 (Donor), set Max HWIDs > 0 (e.g., 5) for a client and copy their subscription link.
3. On Node 1 (Master), navigate to Outbounds -> Subscriptions -> Outbound Subscriptions.
4. Add a new outbound subscription and paste the link from Node 2.
5. Actual result: The panel fails to fetch the config with Failed to fetch/parse subscription (http 404).

Expected behavior

No response

Actual behavior

No response

Relevant logs

Screenshots

No response

3x-ui version

3.8.5

Xray-core version

No response

How did you install 3x-ui?

install.sh script

Operating system

Ubuntu 24.04

Which parts of the panel are affected?

Subscription (share links / Clash / JSON)

Browser (only if it is a UI bug)

No response

Is the panel behind a reverse proxy or CDN?

None

Before submitting

• I searched existing issues and this bug has not been reported.
• I am running the latest released version of 3x-ui (or have verified the bug still exists on it).
• This bug report is written in English.
• I have redacted any sensitive data (tokens, real domains, client UUIDs).