Validate FOC_DEVNET_BASEDIR is not empty before use

[Copilot]

The FOC_DEVNET_BASEDIR environment variable was used without validating it's non-empty, causing paths to resolve relative to the current directory when set to "" or whitespace.

Changes:

• Added trim().is_empty() check in foc_devnet_home() to fall back to default when env var is empty/whitespace
• Extracted default path logic to closure to eliminate duplication
• Added unit tests for empty string, whitespace, unset, valid path, and tilde expansion cases
• Used serial_test to prevent race conditions in tests that modify global env state

pub fn foc_devnet_home() -> PathBuf {
    let default_path = || {
        dirs::home_dir()
            .unwrap_or_else(|| PathBuf::from("/tmp"))
            .join(".foc-devnet")
    };

    if let Ok(base_dir) = std::env::var("FOC_DEVNET_BASEDIR") {
        if !base_dir.trim().is_empty() {
            PathBuf::from(shellexpand::tilde(&base_dir).as_ref())
        } else {
            default_path()
        }
    } else {
        default_path()
    }
}

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[redpanda-f]

I do not like the fallback to /tmp/..., if it is not set, we should still use ~/.foc-devnet