refactor(config): move inline tests to module

[cyq1017]

Closes #3307

Problem

crates/config/src/lib.rs carries a large inline test module, which increases the production file size and future conflict surface.

Change

Move the config tests into crates/config/src/tests.rs and keep the production module wired through #[cfg(test)] mod tests;. No assertions or production logic were changed.

Verification

• cargo fmt --check
• cargo test -p codewhale-config

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.

[Hmbown]

Thanks @cyq1017 — this config test extraction was useful for #3307. The PR branch was stale/conflicting against the current v0.8.64 integration branch, so I reapplied the same extraction to the current test body rather than cherry-picking outdated content.

I preserved you as the commit author with the canonical noreply identity from .github/AUTHOR_MAP, and the local commit body includes Harvested from PR #3345 by @cyq1017.

I am leaving this PR open until the integration branch is pushed/landed so the visible GitHub state stays honest.