Build mautrix-discord's double puppet server map from its own variables

`matrix_bridge_mautrix_discord_bridge_double_puppet_server_map_default` was
built from `matrix_bridge_beeper_linkedin_homeserver_domain` and
`..._homeserver_address` - a different role's variables, copy-pasted along with
the expression.

No user is affected today: group_vars/matrix_servers points both roles'
`homeserver_address` at `matrix_addons_homeserver_client_api_url` and both
domains at `matrix_domain`, so the rendered value is identical either way. It
only diverges for someone who overrides beeper-linkedin's homeserver settings,
where mautrix-discord would silently follow them.

It does not survive mautrix-discord being used without beeper-linkedin's
defaults in scope, which is how the Molecule scenario found it: with only the
one role loaded, the template fails on the undefined name. The scenario's
neutralising override is dropped in the same commit, so the expression is now
exercised rather than bypassed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

roles/custom/matrix-bridge-mautrix-discord/defaults/main.yml

@@ -181,7 +181,7 @@ matrix_bridge_mautrix_discord_bridge_double_puppet_server_map_default: |-
   {{
     {}
     | combine({
-      matrix_bridge_beeper_linkedin_homeserver_domain: matrix_bridge_beeper_linkedin_homeserver_address
+      matrix_bridge_mautrix_discord_homeserver_domain: matrix_bridge_mautrix_discord_homeserver_address
     })
   }}
 matrix_bridge_mautrix_discord_bridge_double_puppet_server_map_auto: {}