Pass ServeHeaderOptions by value instead of pointer, fine tune httplib tests (#36982)

Pass `ServeHeaderOptions` by value instead of pointer across all call
sites — no nil-check semantics are needed and the struct is small enough
that copying is fine.

## Changes

- **`services/context/base.go`**: `SetServeHeaders` and `ServeContent`
accept `ServeHeaderOptions` (value, not pointer); internal unsafe
pointer cast replaced with a clean type conversion
- **`routers/api/packages/helper/helper.go`**: `ServePackageFile`
variadic changed from `...*context.ServeHeaderOptions` to
`...context.ServeHeaderOptions`; internal variable is now a value type
- **All call sites** (13 files): `&context.ServeHeaderOptions{...}` →
`context.ServeHeaderOptions{...}`

Before/after at the definition level:
```go
// Before
func (b *Base) SetServeHeaders(opt *ServeHeaderOptions) { ... }
func (b *Base) ServeContent(r io.ReadSeeker, opts *ServeHeaderOptions) { ... }
func ServePackageFile(..., forceOpts ...*context.ServeHeaderOptions) { ... }

// After
func (b *Base) SetServeHeaders(opts ServeHeaderOptions) { ... }
func (b *Base) ServeContent(r io.ReadSeeker, opts ServeHeaderOptions) { ... }
func ServePackageFile(..., forceOpts ...context.ServeHeaderOptions) { ... }
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/httplib/serve.go

@@ -87,7 +87,7 @@ func serveSetHeadersByUserContent(w http.ResponseWriter, contentPrefetchBuf []by
 	if setting.MimeTypeMap.Enabled {
 		fileExtension := strings.ToLower(path.Ext(opts.Filename))
 		opts.ContentType = setting.MimeTypeMap.Map[fileExtension]
-		detectCharset = !strings.Contains(opts.ContentType, "charset=")
+		detectCharset = strings.HasPrefix(opts.ContentType, "text/") && !strings.Contains(opts.ContentType, "charset=")
 	}
 
 	if opts.ContentType == "" {