'infra single-project' fails when run after `deploy` for agent_runtime

### Problem

When using **Agent Runtime** (Reasoning Engine), running `agents-cli infra single-project` after an initial `agents-cli deploy` does not work as intended.

`infra single-project` uses terraform, but `deploy` does not and uses an API instead. This means that when `single-project` tries to use terraform to update, it doesn't have visibility into the existing reasoning engine resource already created. So, the `terraform apply` invoked by `single-project` silently provisions a **second** Reasoning Engine instance with the same name.

This leads to:
- Observability environment variables being wired to the new (empty) instance.
- The original (serving) instance remaining without observability configuration.

### Workarounds

See https://github.com/google/agents-cli/blob/main/skills/google-agents-cli-observability/SKILL.md#order-of-operations-for-agent_runtime-deployments

The short version though: either run `single-project` before `deploy` so tracking is properly established, or fix it manually using the above instructions.

Ref: b/504375097

Start with the `agents-cli deploy` and `agents-cli infra single-project` flows, then read the order-of-operations guidance in `skills/google-agents-cli-observability/SKILL.md`. Reproduce the commands in that order and verify that `single-project` updates the existing Reasoning Engine rather than creating a second instance, with observability variables attached to the serving instance.

Written by the indexing model from the issue text.