feat: add smoke-otel-tracing workflow for api-proxy OTEL validation (#3475)

Adds a smoke test workflow that validates OpenTelemetry tracing in the
api-proxy sidecar. Tests OTEL module loading, span creation, token usage
attributes, env var forwarding, and OTLP export through Squid proxy.

The workflow uses local builds (via post-processing) and runs on PRs
touching containers/api-proxy/ or the api-proxy service.

Ref: #3469

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox