fix(site): avoid duplicating bin download headers (#22981)

## Summary
- avoid duplicating preset headers when cachecompress serves compressed
`/bin/*` responses
- add a cachecompress regression test for preset
`X-Original-Content-Length` and `ETag` headers
- strengthen site binary tests to assert those headers stay
single-valued

## Problem
`site/bin.go` sets `X-Original-Content-Length` and `ETag` on the real
response writer before delegating.
`cachecompress` then snapshotted those headers and replayed them with
`Header().Add(...)`, which duplicated them on compressed responses.

For `coder-desktop-macos`, duplicate `X-Original-Content-Length` values
can collapse into a comma-separated string and fail `Int64` parsing,
causing the file size to show as `Unknown`.

## Testing
- `/usr/local/go/bin/go test ./coderd/cachecompress -run
'TestCompressorPresetHeaders|TestCompressorHeadings' -count=1`
- `/usr/local/go/bin/go test ./site -run TestServingBin -count=1`
- `PATH=/usr/local/go/bin:$PATH make lint/go`

## Notes
- Skipped full `make pre-commit` with explicit approval because local
environment/tooling blocked it (Node version/path interaction in
generated site targets, plus missing local tools before setup).

coderd/cachecompress/compress.go

@@ -240,9 +240,7 @@ func (c *Compressor) serveRef(w http.ResponseWriter, r *http.Request, headers ht
 		}
 
 		for key, values := range headers {
-			for _, value := range values {
-				w.Header().Add(key, value)
-			}
+			w.Header()[key] = values
 		}
 		w.Header().Set("Content-Encoding", cref.key.encoding)
 		w.Header().Add("Vary", "Accept-Encoding")

coderd/cachecompress/compress_internal_test.go

@@ -155,6 +155,41 @@ type nopEncoder struct {
 
 func (nopEncoder) Close() error { return nil }
 
+func TestCompressorPresetHeaders(t *testing.T) {
+	t.Parallel()
+
+	logger := testutil.Logger(t)
+	tempDir := t.TempDir()
+	cacheDir := filepath.Join(tempDir, "cache")
+	err := os.MkdirAll(cacheDir, 0o700)
+	require.NoError(t, err)
+	srcDir := filepath.Join(tempDir, "src")
+	err = os.MkdirAll(srcDir, 0o700)
+	require.NoError(t, err)
+	err = os.WriteFile(filepath.Join(srcDir, "file.html"), []byte("textstring"), 0o600)
+	require.NoError(t, err)
+
+	compressor := NewCompressor(logger, 5, cacheDir, http.FS(os.DirFS(srcDir)))
+
+	for range 2 {
+		ctx := testutil.Context(t, testutil.WaitShort)
+		req := httptest.NewRequestWithContext(ctx, "GET", "/file.html", nil)
+		req.Header.Set("Accept-Encoding", "gzip")
+
+		respRec := httptest.NewRecorder()
+		respRec.Header().Set("X-Original-Content-Length", "10")
+		respRec.Header().Set("ETag", `"abc123"`)
+
+		compressor.ServeHTTP(respRec, req)
+		resp := respRec.Result()
+
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+		require.Equal(t, []string{"10"}, resp.Header.Values("X-Original-Content-Length"))
+		require.Equal(t, []string{`"abc123"`}, resp.Header.Values("ETag"))
+		require.NoError(t, resp.Body.Close())
+	}
+}
+
 // nolint: tparallel // we want to assert the state of the cache, so run synchronously
 func TestCompressorHeadings(t *testing.T) {
 	t.Parallel()