fix(frontend-proxy): pass CHATBOT_HOST/PORT in base compose so Envoy renders (#3570)

The chatbot upstream cluster was added to the base envoy.tmpl.yaml, but the
frontend-proxy entrypoint runs `envsubst < envoy.tmpl.yaml` using only the
container's own environment, and CHATBOT_HOST/CHATBOT_PORT were injected into
that environment solely by the optional compose.agent.yaml patch.

Whenever the proxy renders its config without those vars present, envsubst
leaves the chatbot socket address empty, so Envoy fails bootstrap validation
(StaticResources.Clusters[10] ... SocketAddress.Address: value length must be
at least 1 characters) and the proxy never starts. That takes down all traffic
through the proxy and was failing the telemetry tests on main.

Pass CHATBOT_HOST/CHATBOT_PORT to frontend-proxy in the base compose.yaml so a
valid address is always rendered, regardless of whether the agent stack is
layered on. The compose.agent.yaml patch keeps its `depends_on: chatbot`.

Assisted-by: Claude Opus 4.8

Signed-off-by: Shenoy Pratik Gurudatt <4348487+ps48@users.noreply.github.com>
Co-authored-by: Shenoy Pratik Gurudatt <4348487+ps48@users.noreply.github.com>

Author: ps48

CHANGELOG.md

@@ -7,6 +7,12 @@ the release.
 
 ## Unreleased
 
+* [frontend-proxy] Pass `CHATBOT_HOST`/`CHATBOT_PORT` to the frontend-proxy in
+  the base compose file. The chatbot upstream cluster lives in the base
+  `envoy.tmpl.yaml` and `envsubst` runs in-container, so without these vars the
+  proxy rendered an empty chatbot address and failed Envoy bootstrap validation
+  whenever the agent stack was not layered on
+  ([#3570](https://github.com/open-telemetry/opentelemetry-demo/pull/3570))
 * [llm] Increase `llm` service memory limit from 50M to 100M to prevent a
   startup restart loop caused by the container exceeding its memory limit
   ([#2944](https://github.com/open-telemetry/opentelemetry-demo/issues/2944))

compose.yaml

@@ -348,6 +348,8 @@ services:
       - FLAGD_UI_PORT
       - TELEMETRY_DOCS_HOST
       - TELEMETRY_DOCS_PORT
+      - CHATBOT_HOST
+      - CHATBOT_PORT
     healthcheck:
       test: ["CMD", "bash", "-c", "echo > /dev/tcp/localhost/${ENVOY_PORT}"]
       start_period: 10s