fix(config): keep path suffix out of project overrides

[Hmbown]

Summary

• prevent repo-local project config from overriding provider path_suffix
• keep path_suffix aligned with the existing project-config policy that ignores endpoint routing fields
• extend the project-override regression test to cover path_suffix

Verification

• cargo test -p codewhale-config --locked project_merge_denies_credentials_endpoints_and_provider_selection -- --nocapture
• cargo test -p codewhale-tui --bin codewhale-tui --locked api_url_with_suffix -- --nocapture
• cargo fmt --all -- --check
• git diff --check

Credit

Follow-up hardening for the provider path-suffix feature from #2558, with review coverage noted on related PRs #2506/#2508.

[greptile-apps]

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[gemini-code-assist]

Code Review

This pull request removes the merging of path_suffix from project-level provider configurations in merge_project_provider_config. It also adds corresponding test cases to verify that the base configuration's path_suffix is not overwritten by the project configuration. No review comments were provided, so there is no feedback to address.

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