refactor: extract validation and socat cleanup from stop.rs handle

[mukeshblackhat]

Summary

• Extracted validate_stoppable — pure function that checks if an instance status allows stopping
• Extracted kill_instance_socat_processes — kills socat PIDs and clears them in the DB
• Updated handle to call both helpers instead of inline logic
• Added 11 unit tests (8 for validation, 3 for socat cleanup)
• #[allow(clippy::cognitive_complexity, clippy::too_many_lines)] kept on handle — still triggers both lints after extraction (follow-up ticket)

What was there before

handle was ~220 lines with status validation (lines 64-88) and socat cleanup (lines 200-216) inlined. The status checks were a chain of if statements comparing against individual InstanceStatus variants.

What changed

Single file: coast-daemon/src/handlers/stop.rs

Extracted functions:

Function	Type	What it does
validate_stoppable(status, name)	Pure, sync	Returns Ok for Running/CheckedOut/Idle, Err for Stopped and transitional states
kill_instance_socat_processes(db, project, name)	Sync	Iterates port allocations, kills socat PIDs, clears them in DB

Deviations from ticket suggestions

Ticket suggested	What we did	Why
Idle grouped with Enqueued (not stoppable)	Idle grouped with Running/CheckedOut (stoppable)	Original code never rejected Idle — it fell through to the stoppable path. coast archive depends on this: handle_archive calls stop::handle for Idle instances. Rejecting Idle broke test_handle_archive_stops_only_running_checked_out_and_idle_instances (expected 3 stopped, got 2). Fixed by preserving the original behavior.
kill_instance_socat_processes as async fn	Made it fn (sync)	All operations inside are synchronous — DB reads via StateDb and process signals via kill_socat. StateDb is not Send, so marking the function async causes compilation errors. The ticket's suggestion was reasonable but doesn't work with the current type constraints.

Test plan

Run new tests

# 17 tests pass (6 existing + 11 new)
cargo test -p coast-daemon -- stop::tests

Verify archive tests still pass

# This test caught the Idle behavioral regression — must pass
cargo test -p coast-daemon -- archive

Run lint and full tests

make lint    # zero warnings
make test    # all pass

Verify behavior is unchanged

# coast stop should work for Running, CheckedOut, and Idle instances
# coast stop should error for Stopped and transitional states
# coast archive should stop all Running + CheckedOut + Idle instances

Closes #122

[jamiesunderland]

Looks great. Thanks @mukeshblackhat