llm: return concrete client types from provider constructors

Follow the Go idiom "return concrete types, accept interfaces": the
provider constructors newOpenAI, newAnthropic, newOpenRouter, newOllama
and their *WithTimeout variants now return their concrete *Client value
types instead of the Client interface. The provider factories registered
in the registry still return Client, so NewFromConfig and the registry
keep working unchanged.

Updated comments to explain the reasoning, dropped the now-redundant
type assertions in tests, and switched the anthropic Streamer-capability
tests to assert via an explicit Client interface value.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 11 insertions, 9 deletions

diff --git a/internal/llm/anthropic_test.go b/internal/llm/anthropic_test.go
index 2459064..4e66d48 100644
--- a/internal/llm/anthropic_test.go
+++ b/internal/llm/anthropic_test.go
@@ -59,7 +59,7 @@ func TestAnthropicChat_Success(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil).(anthropicClient)
+	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil)
 	response, err := c.Chat(context.Background(), []Message{
 		{Role: "user", Content: "Hello"},
 	})
@@ -100,7 +100,7 @@ func TestAnthropicChat_APIError(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "invalid-key", nil).(anthropicClient)
+	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "invalid-key", nil)
 	_, err := c.Chat(context.Background(), []Message{
 		{Role: "user", Content: "Hello"},
 	})
@@ -128,7 +128,7 @@ func TestAnthropicChat_EmptyResponse(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil).(anthropicClient)
+	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil)
 	_, err := c.Chat(context.Background(), []Message{
 		{Role: "user", Content: "Hello"},
 	})
@@ -163,7 +163,7 @@ func TestAnthropicChat_WithTemperature(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil).(anthropicClient)
+	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil)
 	_, err := c.Chat(context.Background(), []Message{
 		{Role: "user", Content: "Hello"},
 	}, WithTemperature(0.5))
@@ -190,7 +190,9 @@ func TestAnthropicStream_Success(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	c := newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil)
+	// newAnthropic now returns the concrete anthropicClient; assert via a Client
+	// interface value to keep verifying the optional Streamer capability.
+	var c Client = newAnthropic(srv.URL, "claude-3-5-sonnet-20241022", "test-key", nil)
 	streamer, ok := c.(Streamer)
 	if !ok {
 		t.Fatalf("Anthropic client does not implement Streamer interface")
@@ -213,7 +215,7 @@ func TestAnthropicStream_Success(t *testing.T) {
 }
 
 func TestAnthropicStream_NoAPIKey(t *testing.T) {
-	c := newAnthropic("https://api.anthropic.com/v1", "claude-3-5-sonnet-20241022", "", nil)
+	var c Client = newAnthropic("https://api.anthropic.com/v1", "claude-3-5-sonnet-20241022", "", nil)
 	streamer, ok := c.(Streamer)
 	if !ok {
 		t.Fatalf("Anthropic client does not implement Streamer interface")
@@ -238,21 +240,21 @@ func TestAnthropicClient_Name(t *testing.T) {
 
 func TestAnthropicClient_DefaultModel(t *testing.T) {
 	model := "claude-3-opus-20250219"
-	c := newAnthropic("https://api.anthropic.com/v1", model, "test-key", nil).(anthropicClient)
+	c := newAnthropic("https://api.anthropic.com/v1", model, "test-key", nil)
 	if c.DefaultModel() != model {
 		t.Fatalf("expected '%s', got '%s'", model, c.DefaultModel())
 	}
 }
 
 func TestAnthropicClient_DefaultBaseURL(t *testing.T) {
-	c := newAnthropic("", "claude-3-5-sonnet-20241022", "test-key", nil).(anthropicClient)
+	c := newAnthropic("", "claude-3-5-sonnet-20241022", "test-key", nil)
 	if c.baseURL != "https://api.anthropic.com/v1" {
 		t.Fatalf("expected default base URL, got '%s'", c.baseURL)
 	}
 }
 
 func TestAnthropicClient_DefaultModel_Empty(t *testing.T) {
-	c := newAnthropic("https://api.anthropic.com/v1", "", "test-key", nil).(anthropicClient)
+	c := newAnthropic("https://api.anthropic.com/v1", "", "test-key", nil)
 	if c.defaultModel != "claude-3-5-sonnet-20240620" {
 		t.Fatalf("expected default model, got '%s'", c.defaultModel)
 	}