feat: add native Ollama provider support via OLLAMA_HOST env var

- OLLAMA_HOST takes priority over OPENAI_BASE_URL for local Ollama instances
- No API key required; placeholder token used for Authorization header
- Model names like 'qwen3:8b' bypass strict provider/model syntax validation
- detect_provider_kind() checks OLLAMA_HOST first in routing cascade
- ProviderClient dispatch uses from_ollama_env() when OLLAMA_HOST is set
- Updated USAGE.md and docs with OLLAMA_HOST as preferred env var
- Added OLLAMA_CONFIG constant and from_ollama_env() to openai_compat
- Added test_ollama_host_bypasses_model_validation unit test
- Supersedes PR #3213 (which had a duplicate if-let bug in mod.rs)

rust/crates/api/src/providers/mod.rs

@@ -351,6 +351,11 @@ fn looks_like_local_openai_model(model: &str) -> bool {
 
 #[must_use]
 pub fn detect_provider_kind(model: &str) -> ProviderKind {
+    // OLLAMA_HOST takes priority: if set, route all models through the local
+    // OpenAI-compatible endpoint regardless of model name or other env vars.
+    if std::env::var_os("OLLAMA_HOST").is_some() {
+        return ProviderKind::OpenAi;
+    }
     let resolved_model = resolve_model_alias(model);
     if let Some(metadata) = metadata_for_model(&resolved_model) {
         return metadata.provider;