mmrs: at command passthrough

[cachebag]

expose modem at command interface for debugging and advanced use.

api

pub async fn at_command(&self, cmd: &str, timeout_secs: u32) -> Result<String>;

wraps Modem.Command(cmd, timeout).

use cases

• query modem firmware: AT+CGMR
• check registration: AT+CREG?
• signal strength: AT+CSQ
• vendor-specific diagnostics

safety

document that this is a low-level escape hatch. incorrect commands can disrupt modem state.

timeout handling

mm has its own timeout; if exceeded, returns an error. surface this as ModemError::Timeout.