fix(e2e): make mock host ports overridable

[lakhansamani]

Why

The e2e stack publishes five fixed host ports — 4000, 4001, 4100, 4200, 4300. If any is in use on the developer's machine, the entire suite dies at startup:

Error response from daemon: Ports are not available: exposing port TCP
0.0.0.0:4100 -> 0.0.0.0:0: listen tcp 0.0.0.0:4100: bind: address already in use

Hit this for real while running make e2e-playground — an unrelated local process held 4100.

There was no way out short of editing the committed compose file. docker-compose.override.yml does not work here: both make targets pass an explicit -f, which suppresses Compose's automatic override discovery — so the documented escape hatch silently does nothing.

What

Each published port becomes ${SERVICE_HOST_PORT:-<default>}:

SMS_SINK_HOST_PORT=4102 make e2e-playground

Variable	Default	Service
MOCK_OAUTH_HOST_PORT	4000	mock-oauth
MOCK_SAML_IDP_HOST_PORT	4001	mock-saml-idp
SMS_SINK_HOST_PORT	4100	sms-sink
WEBHOOK_SINK_HOST_PORT	4200	webhook-sink
CIMD_CLIENT_HOST_PORT	4300	cimd-client

Fixed all five rather than only sms-sink: it is one defect with five instances, and fixing the one port that happened to collide today would leave the next developer to rediscover it on 4000.

Why this is safe

Nothing inside the stack uses the host publication. Every internal reference is over the compose network (SMS_SINK_BASE_URL: http://sms-sink:4100), the mock listens on its container port unchanged, and both the Playwright and Go SDK runners execute inside that network. The host mapping exists only for a human wanting to curl a sink.

Verified

$ docker compose -f e2e-playground/docker-compose.yml config | grep published
  4000  4001  4100  4200  4300          # defaults unchanged

$ SMS_SINK_HOST_PORT=4102 docker compose ... config | grep published
  4000  4001  4102  4200  4300          # only the overridden one moves

The suites themselves were verified green on this stack in the same session: Playwright 98 passed, Go SDK e2e 14 passed.