Ensure we respect Context timeouts in all function calls

[ehdr]

Note

Propagates Context cancellation/timeouts through HTTP requests and polling loops across images, invocations, queues, and sandboxes; adds a gRPC test validating deadline behavior.

• Context propagation and cancellations:
  • Add ctx.Err() checks in long-running/polling loops: image.Build, pollFunctionOutput, queue.get/put/Iterate, Sandbox.Wait, SandboxService.List, ContainerProcess.Wait.
• HTTP requests now context-aware:
  • Use http.NewRequestWithContext for blob PUT upload and GET download in function.go and invocation.go.
• Tests:
  • Add modal-go/test/grpc_test.go with bufconn gRPC server to verify client respects context deadlines (timeout vs completes before deadline).

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