fix(go): stop replacing host process signal handlers during install

[WellMafra]

What

• Remove the signal.Notify(SIGINT/SIGTERM/SIGKILL/SIGQUIT) block and its cancellation goroutine from the native install path; RunWithContext now receives a plain context.Background()
• Add HelmSignalHandlerTest, a black-box test that runs a client-only install in a child JVM, sends it SIGTERM via Process.destroy(), and asserts it exits with code 143 within 15s (disabled on Windows)

Why

• Fixes Go runtime signal handlers permanently replace the host JVM's SIGINT/SIGTERM handlers after install #409: in a c-shared library the first signal.Notify call makes the in-process Go runtime permanently replace the host process' OS-level SIGINT/SIGTERM handlers. Embedding JVMs then ignore SIGTERM — shutdown hooks never run and Kubernetes pods hang until the kubelet SIGKILLs them after terminationGracePeriodSeconds
• The cancellation that signal.Notify enabled was never reachable for embedders anyway (no CLI context), and each call also leaked a goroutine blocked on the signal channel; SIGKILL in the list was a no-op since it cannot be caught
• upgrade.go reaches this same path via Install() when the release doesn't exist, so upgrade --install is covered too

Verified: the new test fails against the previous native library (child JVM ignores SIGTERM and times out) and passes with this change; HelmInstallTest (19 tests) still green; gofmt/go vet clean.