feat: auto-enable path-style addressing when a custom endpoint is set

When AWS_ENDPOINT_URL_S3 or AWS_ENDPOINT_URL is set (typically because
the runtime is pointing at a local MinIO/S3-compatible endpoint),
enable path-style addressing on the S3 client. Without this
requests fail because MinIO doesn't implement virtual-hosted style
addressing out of the box.

Production deployments leave those env vars unset and continue talking
to real AWS S3 with virtual-hosted style, so no behaviour change there.

Both New() and NewS3() share a s3ClientOptions helper that applies the
toggle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

s3_test.go

@@ -40,6 +40,35 @@ func (m *mockPresigner) PresignGetObject(ctx context.Context, params *s3.GetObje
 	return m.presignFunc(ctx, params, optFns...)
 }
 
+// Test path-style toggle
+
+func TestS3ClientOptions_PathStyleTogglesOnCustomEndpoint(t *testing.T) {
+	cases := []struct {
+		name     string
+		envVar   string
+		value    string
+		expected bool
+	}{
+		{name: "no env var → virtual-hosted", envVar: "", expected: false},
+		{name: "AWS_ENDPOINT_URL_S3 set → path-style", envVar: "AWS_ENDPOINT_URL_S3", value: "http://minio:9000", expected: true},
+		{name: "AWS_ENDPOINT_URL set → path-style", envVar: "AWS_ENDPOINT_URL", value: "http://minio:9000", expected: true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("AWS_ENDPOINT_URL", "")
+			t.Setenv("AWS_ENDPOINT_URL_S3", "")
+			if tc.envVar != "" {
+				t.Setenv(tc.envVar, tc.value)
+			}
+			opts := s3.Options{}
+			s3ClientOptions(&opts)
+			if opts.UsePathStyle != tc.expected {
+				t.Fatalf("UsePathStyle = %v, want %v", opts.UsePathStyle, tc.expected)
+			}
+		})
+	}
+}
+
 // Test NewS3 constructor
 
 func TestNewS3(t *testing.T) {