refactor: convert workspacesdk.AgentConn to an interface (#19392)

Fixes https://github.com/coder/internal/issues/907

We convert `workspacesdk.AgentConn` to an interface and generate a mock
for it. This allows writing `coderd` tests that rely on the agent's HTTP
api to not have to set up an entire tailnet networking stack.

Makefile

@@ -636,7 +636,8 @@ GEN_FILES := \
 	coderd/database/pubsub/psmock/psmock.go \
 	agent/agentcontainers/acmock/acmock.go \
 	agent/agentcontainers/dcspec/dcspec_gen.go \
-	coderd/httpmw/loggermw/loggermock/loggermock.go
+	coderd/httpmw/loggermw/loggermock/loggermock.go \
+	codersdk/workspacesdk/agentconnmock/agentconnmock.go
 
 # all gen targets should be added here and to gen/mark-fresh
 gen: gen/db gen/golden-files $(GEN_FILES)
@@ -686,6 +687,7 @@ gen/mark-fresh:
 		agent/agentcontainers/acmock/acmock.go \
 		agent/agentcontainers/dcspec/dcspec_gen.go \
 		coderd/httpmw/loggermw/loggermock/loggermock.go \
+		codersdk/workspacesdk/agentconnmock/agentconnmock.go \
 		"
 
 	for file in $$files; do
@@ -729,6 +731,10 @@ coderd/httpmw/loggermw/loggermock/loggermock.go: coderd/httpmw/loggermw/logger.g
 	go generate ./coderd/httpmw/loggermw/loggermock/
 	touch "$@"
 
+codersdk/workspacesdk/agentconnmock/agentconnmock.go: codersdk/workspacesdk/agentconn.go
+	go generate ./codersdk/workspacesdk/agentconnmock/
+	touch "$@"
+
 agent/agentcontainers/dcspec/dcspec_gen.go: \
 	node_modules/.installed \
 	agent/agentcontainers/dcspec/devContainer.base.schema.json \