Add comprehensive testing for CLI args config override behavior

[brooksmtownsend]

Description

The configuration loading system needs more comprehensive testing to ensure that CLI arguments properly override existing configuration values without replacing present values with empty values.

Location

crates/wash/src/config.rs:153 in the load_config function

Current Behavior

The load_config function implements a hierarchical configuration system with this precedence:

1. Default values
2. Global config (~/.wash/config.json)
3. Local project config (.wash/config.json)
4. Environment variables (WASH_ prefix)
5. Command line arguments (highest priority)

However, there's incomplete testing to ensure CLI args behave correctly when overriding existing configuration.

Problem

The current implementation may have issues where:

• CLI args with empty/default values might replace valid configured values
• Partial CLI config objects might override entire config sections
• Edge cases in the merge behavior aren't properly tested

Proposed Solution

Add comprehensive test coverage for:

1. Selective override testing: Verify CLI args only override specified fields
2. Empty value handling: Ensure empty CLI values don't replace valid config values
3. Nested config merging: Test deep merging behavior for complex config structures
4. Edge cases: Test various combinations of config sources

Example Test Cases

#[test]
fn test_cli_args_selective_override() {
    // Verify that setting only one CLI arg doesn't clear other configured values
}

#[test] 
fn test_cli_args_empty_values() {
    // Verify empty CLI values don't override existing valid config
}

Context

This is critical for ensuring users can rely on CLI arguments working as expected without inadvertently breaking their saved configuration.