Fix PTZ preset list parsing by making Preset.command optional (#326)

[Tahutipai]

Summary

Fixes #326. The camera's reply to /query/ptz/preset lists saved presets with <id>, <name>, and <imageName> only — it does not include a <command> element. The current Preset struct treats command as a required field, so deserialization fails with Custom("missing field \command`"). This error cascades to Unable to parse Payload XML`, causes the camera thread to reconnect, and the ongoing PTZ command is lost. The net effect is that PTZ preset calls silently fail: the camera never moves, and the logs fill with reconnect spam.

The fix makes command default to an empty string on deserialization (#[serde(default)]) while still serializing cleanly for outbound setPos / toPos requests (skip_serializing_if = "String::is_empty").

Root cause

XML returned by the camera for /query/ptz/preset:

<preset>
  <id>0</id>
  <name>BasePoint</name>
  <imageName>preset_0</imageName>
</preset>

Current struct (crates/core/src/bc/xml.rs):

pub struct Preset {
    pub id: u8,
    pub name: Option<String>,
    pub command: String,   // required — but not in list-response XML
}

Same command field is still required when neolink sends setPos / toPos commands to the camera — those paths are unaffected.

Reproduced on

• Reolink E1 (basic), firmware v3.2.0.4858_2508273531
• Previously reported by PTZ preset reporting errog #326 reporter on Reolink Argus PT Ultra, firmware v3.0.0.3492_24050816

Test plan

• Built locally (cargo build --release)
• Deployed patched binary to running Neolink container via bind mount
• Confirmed camera physically moves to saved presets by numeric ID via MQTT
• Confirmed logs no longer show missing field \command`` errors or reconnect loops
• RTSP stream unaffected (runs alongside MQTT via mqtt-rtsp mode)