fix: preserve env values when preload override is false

[Gares95]

Summary

This updates preload override handling so explicit string false values are treated the same as the documented boolean false case.

It also adds focused coverage for environment and CLI preload options.

What changed

• lib/main.js now uses the existing boolean parser when deciding whether populate should overwrite existing values.
• tests/test-config-cli.js now covers:
  • DOTENV_CONFIG_OVERRIDE=false preserving an existing value.
  • dotenv_config_override=false preserving an existing value.
  • DOTENV_CONFIG_OVERRIDE=true continuing to overwrite an existing value.

Why

Preload options arrive as strings from environment variables and CLI arguments. The existing parser already treats strings such as false, 0, no, off, and an empty string as false for other boolean options.

Applying that same parser to override makes preload behavior line up with the documented default while keeping string true behavior unchanged.

Tests run

• Focused Docker validation with npm ci --ignore-scripts:
  tests/test-config-cli.js
  • Result: 6 total, 6 pass.
• Broader Docker validation subset with npm ci --ignore-scripts:
  tests/test-config-cli.js, tests/test-config.js,
  tests/test-populate.js, tests/test-env-options.js,
  tests/test-cli-options.js
  • Result: 89 total, 89 pass.

Risk/compatibility notes

• This is a small behavior change for string-valued override options.
• String true still enables override.
• Boolean true and boolean false behavior remains consistent with the documented option.
• Users who intentionally relied on any non-empty override string enabling override would see false, 0, no, off, and empty-string values treated as false.
• The option extraction helpers continue to return string values; normalization remains at the point where overwrite behavior is decided.

Maintainer notes

• I kept this change scoped to populate so env and CLI option parsing behavior remains unchanged.
• I added coverage at the preload level because that is where string-valued options are user-visible.
• Happy to adjust the test placement if a lower-level populate test would be preferred.