policy: support convert decisions from sessionpolicy

[tonistiigi]

No description provided.

[crazy-max]

Error in this test looks legit: https://github.com/moby/buildkit/actions/runs/20328071722/job/58406954737?pr=6427#step:8:568

=== FAIL: client TestIntegration/slice=3-4/TestSourcePolicySessionConvert/worker=containerd/convert_and_allow (0.82s)
    policy_test.go:744: 
        	Error Trace:	D:/a/buildkit/buildkit/client/policy_test.go:744
        	Error:      	Received unexpected error:
        	            	failed to load cache key: no match for platform in manifest: not found
        	            	github.com/moby/buildkit/util/stack.Enable
        	            		D:/a/buildkit/buildkit/util/stack/stack.go:82
        	            	github.com/moby/buildkit/util/grpcerrors.FromGRPC
        	            		D:/a/buildkit/buildkit/util/grpcerrors/grpcerrors.go:264
        	            	github.com/moby/buildkit/util/grpcerrors.UnaryClientInterceptor
        	            		D:/a/buildkit/buildkit/util/grpcerrors/intercept.go:41
        	            	google.golang.org/grpc.(*ClientConn).Invoke
        	            		D:/a/buildkit/buildkit/vendor/google.golang.org/grpc/call.go:35
        	            	github.com/moby/buildkit/api/services/control.(*controlClient).Solve
        	            		D:/a/buildkit/buildkit/api/services/control/control_grpc.pb.go:88
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		D:/a/buildkit/buildkit/client/solve.go:319
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		D:/a/buildkit/buildkit/vendor/golang.org/x/sync/errgroup/errgroup.go:93
        	            	runtime.goexit
        	            		C:/hostedtoolcache/windows/go/1.25.5/x64/src/runtime/asm_amd64.s:1693
        	            	failed to solve
        	            	github.com/moby/buildkit/client.(*Client).solve.func2
        	            		D:/a/buildkit/buildkit/client/solve.go:321
        	            	golang.org/x/sync/errgroup.(*Group).Go.func1
        	            		D:/a/buildkit/buildkit/vendor/golang.org/x/sync/errgroup/errgroup.go:93
        	            	runtime.goexit
        	            		C:/hostedtoolcache/windows/go/1.25.5/x64/src/runtime/asm_amd64.s:1693
        	Test:       	TestIntegration/slice=3-4/TestSourcePolicySessionConvert/worker=containerd/convert_and_allow
        --- FAIL: TestIntegration/slice=3-4/TestSourcePolicySessionConvert/worker=containerd/convert_and_allow (0.82s)

[tonistiigi]

@crazy-max Forgot windows skip.