controller/vm: add comprehensive unit tests for VM Controller

Add 42 test cases covering:
- State.String() for all states including unknown/negative values
- TerminateVM state transitions (idempotency, success, error, recovery)
- Guard checks for all state-dependent methods with error content validation
- ExecIntoHost validation, success, non-zero exit code, and exec error paths
- DumpStacks with supported/unsupported capabilities and dump error path
- StartTime for all states
- ExitStatus success and error cases (table-driven)
- StartVM idempotency and error states (table-driven)

Follows Go test best practices:
- Table-driven tests with t.Run() and / separator for subtest names
- t.Parallel() on all subtests for concurrent execution
- defer ctrl.Finish() per repo convention (TestLookupVMMEM pattern)
- t.Context() instead of context.Background() (Go 1.24+)
- t.Errorf for independent assertions, t.Fatalf only for nil guards
- Error content validation (not just err != nil)

Signed-off-by: Shreyansh Sancheti <shreyanshjain7174@gmail.com>
Signed-off-by: Shreyansh Sancheti <shsancheti@microsoft.com>

Author: Shreyansh Sancheti

internal/controller/vm/export_test.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package vm
+
+import (
+	"github.com/Microsoft/hcsshim/internal/vm/vmmanager"
+)
+
+// SetManagerForTest configures a Manager with injected dependencies for testing.
+// This is only available in test builds.
+//
+// MUST be called during test setup before any concurrent operations on the Manager.
+func (c *Manager) SetManagerForTest(uvm vmmanager.LifetimeManager, guest GuestManager, state State) {
+	c.uvm = uvm
+	c.guest = guest
+	c.vmState = state
+}

internal/controller/vm/state_test.go

@@ -0,0 +1,33 @@
+//go:build windows
+
+package vm_test
+
+import (
+	"testing"
+
+	vm "github.com/Microsoft/hcsshim/internal/controller/vm"
+)
+
+func TestStateString(t *testing.T) {
+	tests := []struct {
+		state vm.State
+		want  string
+	}{
+		{vm.StateNotCreated, "NotCreated"},
+		{vm.StateCreated, "Created"},
+		{vm.StateRunning, "Running"},
+		{vm.StateTerminated, "Terminated"},
+		{vm.StateInvalid, "Invalid"},
+		{vm.State(99), "Unknown"},
+		{vm.State(-1), "Unknown"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.want, func(t *testing.T) {
+			t.Parallel()
+			if got := tt.state.String(); got != tt.want {
+				t.Errorf("State(%d).String() = %q, want %q", tt.state, got, tt.want)
+			}
+		})
+	}
+}