fix(docs): correct nested agent server entry in Quickstart uv example

[ftnext]

Summary

Fix the uv configuration example in docs/quickstart.md (Step 3 — Connect from an ACP-aware client, Zed section) by restoring the missing "Echo Agent (Python)" nesting under "agent_servers".

Also aligns the JSON snippet structure with the preceding non-uv example so users can copy/paste a valid server entry.

Related issues

N/A

Testing

• docs-only change
• Manually validated the JSON with Zed's Agent Panel

Docs & screenshots

• Updated: docs/quickstart.md
• No screenshots (documentation-only, no UX change).

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.