fix: kill entire CLI process tree on stop/forceStop (Windows)

[rinceyuan]

Summary

Rust is not included - #2292 landed that fix, and this branch takes it as-is.

Fix the process-tree leak when stop() / forceStop() terminates the CLI. On Windows ChildProcess.kill() / Popen.terminate() only ends the root, leaving grandchildren orphaned. On POSIX a SIGTERM-resistant descendant survives after the root exits.

Closes #1804.

Spawn-time isolation

Put the CLI in its own process group so the whole tree can be signalled:

SDK	Mechanism
Node.js	detached: true
Python	start_new_session=True
Go	SysProcAttr{Setpgid: true}
Java / .NET	not needed (tree-kill APIs handle it)

Teardown

Private helpers, no public API change, called from stop() and forceStop():

Platform	Mechanism
Windows	taskkill /T /F /PID
POSIX	kill(-pid, signal) on the process group
Java	ProcessHandle.descendants() collected before the root is signalled
.NET	already uses Kill(entireProcessTree: true) - unchanged

stop() is graceful first and escalates: POSIX sends SIGTERM to the group, waits, then SIGKILLs the group. The escalation is unconditional because the root exiting says nothing about a descendant that ignored SIGTERM.

Windows always uses taskkill /T /F. It has no graceful signal (kill() is TerminateProcess regardless), and /T can only enumerate the tree while the root is alive, so a graceful root close would strand the descendants.

Failure handling

Every helper falls back to the single-process termination it replaced when the tree-wide path is unavailable or fails - missing pid, non-zero taskkill, ESRCH from killpg. Go propagates the error after the fallback also fails; Rust checks the exit status and reaps the root.

Go's killProcessTreeByPid uses the PID from the atomically swapped osProcess, not the mutex-guarded c.process.

Tests

nodejs/test/process_tree_kill.test.ts drives CopilotClient.stop() / forceStop() over a real spawned tree, so removing the tree termination fails them:

1. stop() terminates descendants of the owned runtime
2. forceStop() terminates descendants of the owned runtime
3. stop() reaps a descendant that ignores SIGTERM (POSIX only)
4. External-server connections are left alone

python/test_client.py gains TestKillProcessTree covering the group signal, the taskkill invocation and both fallbacks.

Validation

python -m pytest test_client.py -q                 122 passed   (ruff check/format clean)
npx vitest run test/process_tree_kill.test.ts      3 passed, 1 skipped (Windows)
  same file under WSL Ubuntu                       4 passed
npx tsc --noEmit / eslint                          clean
go vet ./... ; go test .                           clean / ok
  GOOS=linux and GOOS=darwin go build ./...        clean
mvnw test -pl sdk -Dtest=CopilotClientTest         41 passed   (spotless + checkstyle clean)
cargo +nightly fmt --check ; clippy -D warnings    clean
cargo test --all-features --lib                    226 passed

Not run locally: the internal/e2e Go package and the Rust e2e target, which need the replay harness and fail identically on a clean checkout.