feat(config): allow provider chat path suffix

[Hmbown]

Summary

• add provider-scoped path_suffix config for OpenAI-compatible chat completions endpoints
• route chat-completion requests through the override while keeping models, health, and beta FIM URL behavior unchanged
• document the /chat/completions gateway setup in config docs and example config

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p codewhale-config path_suffix
• CARGO_TARGET_DIR=/Volumes/VIXinSSD/whalebro/codewhale-target/issue-2089 cargo test -p codewhale-tui path_suffix
• CARGO_TARGET_DIR=/Volumes/VIXinSSD/whalebro/codewhale-target/issue-2089 cargo test -p codewhale-tui openai_provider_accepts_custom_model_and_base_url
• CARGO_TARGET_DIR=/Volumes/VIXinSSD/whalebro/codewhale-target/issue-2089 cargo test -p codewhale-cli build_tui_command_forwards_resolved_runtime (compile-only; filter matched 0 tests)

Closes #2089
Closes #1874

[gemini-code-assist]

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