Create endpointlogcollections in the chain that owns it

A fresh install could not build its schema. Core revision
7d51_endpoint_logcollections creates endpointlogcollections with a foreign
key onto endpointcommands, but PLUGIN_TABLE_OWNERS lists both tables as
owned by the endpoints plugin, and only the plugin's 0002 revision creates
endpointcommands.

Core migrations run before any plugin chain, and a plugin chain only runs
once that plugin is enabled, which cannot happen before the schema exists.
So there is no ordering in which the core revision can succeed on an empty
database. MySQL rejects it with 1824, 'Failed to open the referenced table
endpointcommands'. That is the first command the README gives an adopting
site, on the branch that exists for sites to adopt.

Nothing caught it because the suite runs on sqlite:///:memory: and SQLite
accepts a foreign key onto a table that does not exist. The failure is
MySQL-only and therefore invisible to every test.

Fix:
- endpoints revision 0025 creates the table in the plugin chain, after
  0002 has created its foreign key target. Guarded, so it is a no-op on
  sites that already applied the core revision.
- 7d51 skips when endpointcommands is absent, which is exactly the fresh
  install case 0025 then covers. Its downgrade is guarded to match.
- EXPECTED_HEAD_REVISION records the new endpoints head.

Verified against real MySQL 8.4, not SQLite: on an empty database the
README sequence now completes, 7d51 stamps without creating anything, and
enabling the endpoints plugin builds endpointlogcollections with
endpointlogcollections_ibfk_2 pointing at endpointcommands.

tests/test_plugin_migrations.py

@@ -123,7 +123,7 @@ EXPECTED_HEAD_REVISION['warranty'] = 'warranty0002proof'
 # endpointcredentials, endpointenrollrequests and endpointmigrationwaves.
 # 0024 adds endpointbootstraptokens: the short-lived, admin-minted tokens behind
 # the one-line bootstrap installer.
-EXPECTED_HEAD_REVISION['endpoints'] = 'endpoints0024bootstraptokens'
+EXPECTED_HEAD_REVISION['endpoints'] = 'endpoints0025logcollections'
 
 # Plugins built after the cutover: their 0001 baseline really creates tables the
 # core chain never owned.