Add path_suffix to ProviderConfigToml and ProviderConfig

Adds an optional path_suffix field that lets users override the API path
for OpenAI-compatible endpoints. When set, the suffix replaces the default
/v1/<path> pattern, enabling use with endpoints that don't accept /v1/
prefixes (e.g. /chat/completions instead of /v1/chat/completions).

Changes:
- ProviderConfigToml (config crate): path_suffix field
- ProviderConfig (tui crate): path_suffix field
- merge_provider_config: propagates path_suffix
- merge_project_provider_config: propagates path_suffix
- api_url: delegates to new api_url_with_suffix function
- api_url_with_suffix: uses suffix when present, skips /v1 versioning
- DeepSeekClient: reads path_suffix from config, passes to URL builder
- config.example.toml: documents the new option
- Tests for the new URL building behavior

Closes #2089

config.example.toml

@@ -273,6 +273,7 @@ max_subagents = 10 # optional (1-20)
 # base_url = "https://your-provider.example/v1"
 # model = "deepseek-ai/DeepSeek-V4-Pro"
 # http_headers = { "X-Model-Provider-Id" = "your-model-provider" } # optional custom request headers
+# path_suffix = "/chat/completions" # override the API path; skips /v1 versioning when set
 
 # NVIDIA NIM-hosted DeepSeek V4 (https://build.nvidia.com)
 [providers.nvidia_nim]