Fix nil pointer dereference in executeAzdCommand test utility

[Copilot]

Addresses potential nil pointer panic when cli.RunCommandWithStdIn fails to start. The function returns nil result on startup failure, but the code unconditionally dereferenced result.Stdout and result.Stderr.

Changes:

• Add nil check before accessing result fields
• Split error handling: err != nil case (uses -1 exit code when result is nil) and defensive result.ExitCode != 0 case
• Fix incorrect doc comment on executeAzdCommandWithExec

Before:

result, err := cli.RunCommandWithStdIn(ctx, "", args...)
output := result.Stdout + result.Stderr  // panic if result is nil
if err != nil || result.ExitCode != 0 {
    // both conditions checked in single block
}

After:

result, err := cli.RunCommandWithStdIn(ctx, "", args...)

var output string
if result != nil {
    output = result.Stdout + result.Stderr
}

if err != nil {
    var exitCode int = -1  // command failed to start
    if result != nil {
        exitCode = result.ExitCode
    }
    // handle error
}

if result != nil && result.ExitCode != 0 {
    // defensive check for non-zero exit without error
}

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