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).

site/site_test.go

@@ -562,7 +562,7 @@ func TestServingBin(t *testing.T) {
 					}
 
 					if tr.wantEtag != "" {
-						assert.NotEmpty(t, resp.Header.Get("ETag"), "etag header is empty")
+						assert.Equal(t, []string{tr.wantEtag}, resp.Header.Values("ETag"), "etag header values did not match")
 						assert.Equal(t, tr.wantEtag, resp.Header.Get("ETag"), "etag did not match")
 					}
 
@@ -570,6 +570,8 @@ func TestServingBin(t *testing.T) {
 						// This is a custom header that we set to help the
 						// client know the size of the decompressed data. See
 						// the comment in site.go.
+						headerValues := resp.Header.Values("X-Original-Content-Length")
+						assert.Len(t, headerValues, 1, "X-Original-Content-Length should have exactly one value")
 						headerStr := resp.Header.Get("X-Original-Content-Length")
 						assert.NotEmpty(t, headerStr, "X-Original-Content-Length header is empty")
 						originalSize, err := strconv.Atoi(headerStr)