fix(backend/copilot-bot): point /unlink button to the new bots page (#13198)

### Why / What / How

**Why:** The Discord `/unlink` command's "Open Settings" button links to
the **old** settings page (`/profile/settings`). Reported by Abhi during
bot testing — it lands users on the deprecated settings UI instead of
the current one.

**What:** Repoint the `/unlink` button (and the no-base-URL fallback
text) to the new `/settings/bots` page, where chat-bot links (DM +
linked servers) are managed and can be removed.

**How:** One-line URL change in `_handle_unlink` (`/profile/settings` →
`/settings/bots`) plus the matching fallback-text wording ("Profile →
Linked accounts" → "Settings → Bots").

> ⚠️ **Depends on #13222** (the new Bots settings page). Merge that
first or the button will land users on a 404.

### Changes 🏗️

- `adapters/discord/commands.py` — `_handle_unlink` button URL →
`/settings/bots`; fallback text updated to "Settings → Bots".
- `adapters/discord/commands_test.py` — assert the button URL ends with
`/settings/bots`; update the no-URL fallback assertion to look for
"Bots".

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] `TestHandleUnlink` asserts the button points at `/settings/bots`
- [x] Full `commands_test.py` suite green (9 passed), `black` + `ruff`
clean