Fix empty VSS URL handling in config files

[Kewe63]

Fixes #161

Summary

• Treat an empty [vss].url from the TOML file as disabled, matching sample-config.toml.
• Preserve CLI precedence: a valid --vss-url still overrides the file, while explicit blank CLI values remain validation errors.
• Add regression coverage for the empty-file sentinel and CLI/file precedence matrix.

Why

The resolver previously preserved url = "" as Some("") and passed it to validate_vss_url, causing startup to fail even though the sample config documents an empty value as VSS disabled.

Verification

• RED before fix: args::tests::empty_vss_url_disables_vss failed with InvalidVssConfig.
• Default-feature argument tests: 19 passed.
• vss-feature argument tests: 19 passed.
• Config tests: 44 passed.
• cargo +1.94.0 clippy --locked --lib -- -D warnings: passed.
• cargo +1.94.0 fmt -- --check: passed.
• git diff --check: passed.

Scope

No network or regtest service was used. The full integration suite was not run because it requires the shared regtest service stack; the change is limited to startup argument resolution and its unit tests.