feat(server): add streaming response helpers for file download

Add ParseRange, StreamFile, StreamRange for full and partial content delivery.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/server/response_test.go

@@ -114,3 +114,65 @@ func TestErrorWithStatus(t *testing.T) {
 		t.Fatalf("expected error 'already exists', got '%s'", resp.Error)
 	}
 }
+
+func TestParseRangeStandard(t *testing.T) {
+	tests := []struct {
+		rangeHeader string
+		fileSize    int64
+		wantStart   int64
+		wantEnd     int64
+		wantErr     bool
+	}{
+		{"bytes=0-1023", 10000, 0, 1023, false},
+		{"bytes=1024-", 10000, 1024, 9999, false},
+		{"bytes=-1024", 10000, 8976, 9999, false},
+		{"bytes=0-0", 10000, 0, 0, false},
+		{"bytes=9999-", 10000, 9999, 9999, false},
+	}
+	for _, tt := range tests {
+		start, end, err := ParseRange(tt.rangeHeader, tt.fileSize)
+		if (err != nil) != tt.wantErr {
+			t.Errorf("ParseRange(%q, %d) error = %v, wantErr %v", tt.rangeHeader, tt.fileSize, err, tt.wantErr)
+			continue
+		}
+		if !tt.wantErr {
+			if start != tt.wantStart || end != tt.wantEnd {
+				t.Errorf("ParseRange(%q, %d) = (%d, %d), want (%d, %d)", tt.rangeHeader, tt.fileSize, start, end, tt.wantStart, tt.wantEnd)
+			}
+		}
+	}
+}
+
+func TestParseRangeInvalidAndMultiPart(t *testing.T) {
+	tests := []struct {
+		rangeHeader string
+		fileSize    int64
+	}{
+		{"", 10000},
+		{"bytes=9999-0", 10000},
+		{"bytes=20000-", 10000},
+		{"bytes=0-100,200-300", 10000},
+		{"bytes=0-100, 400-500", 10000},
+		{"bytes=", 10000},
+		{"chars=0-100", 10000},
+	}
+	for _, tt := range tests {
+		_, _, err := ParseRange(tt.rangeHeader, tt.fileSize)
+		if err == nil {
+			t.Errorf("ParseRange(%q, %d) expected error, got nil", tt.rangeHeader, tt.fileSize)
+		}
+	}
+}
+
+func TestParseRangeEdgeCases(t *testing.T) {
+	start, end, err := ParseRange("bytes=0-99999", 10000)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if end != 9999 {
+		t.Errorf("end = %d, want 9999 (clamped to fileSize-1)", end)
+	}
+	if start != 0 {
+		t.Errorf("start = %d, want 0", start)
+	}
+}