refactor(dgw): tie test processes to their config

[Copilot]

start_dgw returned a process independent of the handle owning its temporary configuration directory.
Replace the documented lifetime requirement with a compiler-enforced borrow.

• Ownership: Add DgwConfigHandle::start() returning DgwProcess<'_>, with an explicit destructor that keeps the configuration borrowed through guard destruction.
• Compatibility: Expose start_kill() and wait(); preserve configuration environment, null output, kill-on-drop, listener readiness, and timeout cleanup.
• Migration: Replace all three callers and remove the free function.
• Regression coverage: Enable doctests and add a compile-fail example rejecting premature configuration destruction.

let mut process = config_handle.start().await?;
process.start_kill()?;
process.wait().await?;