fix(showcase/ms-agent-python): drop trailing slash on subpath HttpAgent URLs

The mcp-apps and voice-demo HttpAgent URLs had a trailing slash
(`${AGENT_URL}/mcp-apps/`, `${AGENT_URL}/voice/`), but the FastAPI
backend in agent_server.py mounts those agents at `/mcp-apps` and
`/voice` exactly. Posting to the trailing-slash URL triggers FastAPI's
default `redirect_slashes` 307, which drops the SSE streaming body and
surfaces in the runtime as
`RUN_ERROR: fetch failed (INCOMPLETE_STREAM)` for every pill click on
the deployed ms-agent-python showcase.

Reproduced live against showcase-ms-agent-python-production. Every
other ms-agent-python HttpAgent URL (`/hitl-in-app`,
`/headless-complete`, `/multimodal`, `/agent-config`, etc.) already
uses no trailing slash and works fine, confirming the trailing slash
is the only delta.