Components here contact a homeserver while starting up and exit if it is
unreachable, so nearly every scenario will need one. Standing up a real
Synapse per role would dominate the run and drag in Postgres, and these
scenarios are not testing Synapse.

The stub answers the handful of endpoints components touch during startup
with the blandest plausible response, and is deliberately permissive: an
unrecognised path returns {} rather than 404, because the goal is to get the
component past its startup checks. It is not an authentication check or a
room state machine, and a scenario should not assert *about* it - if one
starts needing it to behave like a real homeserver, that scenario has
outgrown what these tests are for.

matrix-alertmanager-receiver now includes it instead of carrying its own
inline copy. Verified green afterwards.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Three things that would not have scaled to 70 roles:

- The Python and Ansible dependency pins were about to be copied into every
  role. They now live once in molecule-shared/, which scenarios reference
  relatively, so they cannot drift apart.
- The helper container images used for probing were hardcoded inline. They
  are pinned once in molecule-shared/vars.yml, carry `# renovate:`
  annotations, and a custom manager in .github/renovate.json keeps them
  current - verified with a local Renovate dry run, which offers
  curl 8.11.1 -> 8.21.0 and python 3.13 -> 3.14-alpine. Seventy invisible
  hardcodes is the blindness class we have been removing elsewhere.
- Running a scenario meant knowing the venv and cd incantation. `just
  molecule <role>` does it, and with no argument lists the roles that have
  a scenario.

Molecule is deliberately not wired into prek: a run takes minutes, pulls
images and needs Docker, which is fine on request and not fine per commit.

docs/molecule-testing.md covers how to run and write these, including the
four things a role here needs that a standalone role does not. AGENTS.md
points at it rather than carrying the detail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Not for merging as-is. One role's scenario plus the workflow that would
make per-role testing affordable here, so we can see how it behaves before
deciding whether to do the other 69.

Unlike the MASH repositories, one repository holds every role, so running
everything on every push is not an option. The workflow's first job works
out which roles a push touched and builds the matrix from that; a change to
docs, or to a role with no scenario yet, runs nothing.

Three things this role needed that a MASH role does not:

- The variables matrix-base would supply (matrix_base_data_path,
  matrix_domain, matrix_user_name, matrix_group_name and the uid/gid) have
  to be provided by the scenario, and the user and group have to exist
  before the role's file tasks run.
- The service contacts the homeserver while starting up - it fetches
  /_matrix/client/v3/joined_rooms to resolve its room mapping and exits 1
  if that fails - so prepare.yml stands up a stub homeserver. Most roles
  here are bridges and bots, so this is likely the rule rather than the
  exception.
- verify.yml runs as its own play, where role defaults are out of scope, so
  the paths it reads are pinned in molecule.yml. The version is deliberately
  NOT pinned: it is read from defaults/main.yml so the assertion compares
  the running image against what the role ships.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>