fix: preserve unknown LAN camera live-view status

[jlapenna]

Fixes #1.

Problem and impact

Missing camera telemetry was reported as an explicit disabled LAN live-view setting. That can mislead operators and agents into unnecessary setting changes when the printer has simply not advertised the route.

Change

Preserve missing, null, empty or malformed ipcam.rtsp_url as unknown: JSON camera_lan_liveview: null, and human output liveview=unknown. Explicit disable remains false; a nonempty advertised route remains true, without claiming that capture succeeds. The existing field name is retained and the nullable public API change is documented in CHANGELOG.

This is a read-only status-contract correction, not a camera-access, signed-dispatch or physical-printing fix. It changes no printer setting or print approval/preflight behavior and is ready for independent review of that narrow scope.

Verification

• Reverified on 2026-10-06 in an isolated worktree at unchanged PR head 1c8be5488dd7c6e70be260d8154bd8a0e37d8824, against upstream main 0b822df5c9434cb5838e9a1a371eed640b702992.
• make ci passed: lint (zero issues), vet, all package race tests, and CLI build.
• Independently ran the pinned goimports and gofumpt checks; both produced no formatting differences. This independently checks formatting because the Makefile's /bin/sh pipeline emits a read: arg count diagnostic on this host.
• git diff --check origin/main..HEAD passed. Seven telemetry-state regression cases assert the serialized JSON contract, and existing status/command tests passed.
• No live printer test, camera setting change, physical action or source change during this review follow-up.

The earlier draft hold for downstream printing investigation is removed: that investigation is separate and does not change the correctness or scope of this telemetry fix. Maintainer review/acceptance and upstream merge are still required.

[jlapenna]

@voska This narrow telemetry fix is now ready for review; the earlier draft hold was for a separate downstream printing investigation. I reverified the unchanged head with make ci (lint, vet, every package race test and build), independent pinned formatting checks, and diff whitespace validation. The updated description makes the nullable public-field change and scope explicit. It does not change printer settings or claim to fix camera connectivity. Please review the unknown/false/true contract and merge if acceptable.

[voska]

Thanks Joe. Reviewed and verified locally: make ci is green at 1c8be54, and the new table test fails on main for all five unknown cases. That includes rtsp_url: false, which main reported as true, so this fixes a second bug too. Making it nullable matches dev_mode/protections, and pre-1.0 it's fine as a minor-version contract change. Merging.