fix(boxes): Test Connection on existing-box edit form

[sarg3nt]

Summary

• Test Connection on the box edit form was failing on existing boxes because the JS sent a non-existent server_id form field. The backend already supports a "look up by box_id, decrypt the stored key, run probe server-side" recovery path at haproxy_config.go:427-449 — it just never received the box_id needed to trigger it.
• Fix: rename the form field the JS sends (server_id → box_id) and read from the existing readonly box_id input that the edit form already renders. No backend changes needed.
• Phase 0 of the rotation plan in Auto agent API Key rotation #72; landed independently so the visible bug clears now and the multi-key rotation work that follows starts from a working Test Connection baseline.

Test plan

• On a freshly-added box, Test Connection succeeds (regression check — this code path was already working).
• On an existing box opened via the edit form without typing anything into the API Key field, Test Connection succeeds (the actual bug being fixed).
• On an existing box where the operator pastes a new API key into the field, Test Connection uses the typed key (form takes precedence over stored key when both are present — backend behavior unchanged).
• On an existing box that has no API key configured at all (rare), Test Connection returns the existing "API key is required" error rather than silently using a stale lookup.
• make templ-generate && make build clean.

Related

• Closes the "cannot test connection of an existing box" bug noted in Auto agent API Key rotation #72.
• Adjacent to Test Connection probe 404s on boxes without HAProxy; audit all probes for server-side API key handling #111 (Test Connection 404 on non-HAProxy boxes + server-side API key handling audit) — they touch the same surface but address different failure modes; both can land independently.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes the Box edit form’s “Test Connection” path by sending the existing box_id to the backend so it can load and decrypt the stored API key server-side.

Changes:

• Updates the Test Connection form payload from server_id to box_id.
• Adds an inline comment explaining why edit-mode connection tests need box_id.