Centralize the hand-rolled ateapipb.ControlClient test fake

[sairajp-rewind]

PROBLEM

Five test files hand-roll a fake ateapipb.ControlClient, across four packages. #907 adds a sixth,
which is what surfaced this. That PR should land as it stands; consolidating these is follow-up
cleanup, not a prerequisite for it.
Surveyed at 3f132c6d (main, 2026-08-17). Links are permalinks pinned to that commit so the line numbers can't drift; the type names are the durable anchor.

type	package	location	RPCs stubbed
mockControlClient	controllers	actortemplate_controller_test.go:97	CreateAtespace, CreateActor
healthControlClient	router	health_test.go:41	ListActors
mockClient	ingress	ingress_test.go:39	ResumeActor
resumerMockClient	ingress	resumer_test.go:32	ResumeActor
egressMockClient	egress	egress_test.go:197	GetActor

The sixth, not yet merged: fakeControlClient in internal/benchmarking/boomer/glutton/fixture_test.go, package glutton, stubbing CreateAtespace, CreateActor, ResumeActor, SuspendActor, DeleteActor.

The two ingress ones live in the same package: the same one-RPC ResumeActor fake under two names. Their struct definitions are identical; only the method bodies differ, and resumerMockClient nil-guards resumeFn where mockClient calls it unconditionally.

All six are unexported and live in _test.go files, so none is importable. All six use the same idiom: embed ateapipb.ControlClient as an anonymous field and override the handful of RPCs the test needs, so calls to any RPC no fake overrides compile and nil-panic at call time.

The union across all six is seven RPCs: CreateAtespace, CreateActor, ResumeActor, SuspendActor, DeleteActor, GetActor, ListActors. ControlClient declares 28, so one embedding fake needs pluggable func fields for just those seven and inherits the rest.

PROPOSAL

An exported fake in a non-_test.go file under repo-root internal/, following the precedent
already set by internal/volume/mock.go:52 (MockVolumePlugin, with NewMockVolumePlugin() at
:55), which cmd/ateapi/internal/controlapi/functional_test.go and controlapi/volumes_test.go already call cross-package.

Placement has to be repo-root internal/: cmd/<app>/internal/... is importable only from under
that cmd/<app>, and pkg/proto/ateapipb is generated. pkg/ would work mechanically but makes a
test fake part of the public API surface. Put it in a dedicated package rather than an existing production one, so only tests link it and it never reaches a shipped binary.

[maxsmythe]

Although providing a fake for downstream consumers to use to write their own tests could be a nice convenience feature for them.