docs(anthropic-messages): replace hardcoded vLLM IP with a configurable env var

[jordigilh]

Gap

demos/anthropic-messages/README.md hardcodes a vLLM endpoint IP in its prerequisites:

vLLM endpoint at 10.0.0.99:8000 (update passthrough.yaml / transform.yaml for your setup)

This forces anyone running the demo (including in front of a customer) to manually edit two YAML config files rather than just setting an environment variable. Low risk, but it's rough edges on a table-stakes API-compatibility demo (Anthropic /v1/messages passthrough + transform).

Proposed fix

• Template the vLLM endpoint in passthrough.yaml / transform.yaml from an env var (e.g. VLLM_ENDPOINT), consistent with how other demos in this repo parameterize environment-specific values.
• Update the README's Quick Start / Prerequisites accordingly.

Documentation/config only — no Praxis code changes.