Require an exit code before exec reports success

[MiguelsPizza]

Non-interactive hypeman exec returns 0 when the WebSocket closes without an {"exitCode":N} message (dropped connection, normal close, malformed or null code), and a received nonzero code can also lose the final select to the reader's done channel. This makes the exit code message required: anything else returns an error instead of success. The interactive TTY path is unchanged.

Tests: go test -count=1 ./pkg/cmd/ -run TestRunExecNonInteractive. On main all four TestRunExecNonInteractiveRequiresExitCode cases fail with "An error is expected but got nil"; they pass with the fix.

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Note

Medium Risk
Changes CLI exit semantics for non-interactive exec (scripts/CI may now fail on connection issues), but tightens correctness and does not alter the interactive path.

Overview
Non-interactive hypeman exec no longer reports success unless the server sends a valid {"exitCode":N} text message.

runExecNonInteractive drops the doneCh fallback that returned exit 0 when the WebSocket closed cleanly, and treats any read error (including normal close or a dropped connection) as failure with "websocket closed before receiving an exit code". Exit codes are parsed with a nullable *int so malformed JSON, null, or missing values do not count as success. Interactive TTY exec behavior is unchanged.

Adds WebSocket harness tests for propagating a nonzero exit code and for the cases that must now error instead of exiting 0.

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