[Fix] re_cache: Always use raw mode for SARAWBODY regexps

[Fix] re_cache: Always use raw mode for SARAWBODY regexps

The parsed content is transfer-decoded (base64/QP) but NOT charset-converted,
so it may contain non-UTF-8 data even when IS_TEXT_PART_UTF is true.

Using dynamic raw flag based on IS_TEXT_PART_UTF was incorrect because that
flag indicates whether utf_content is valid UTF-8, not whether parsed content
is valid UTF-8.

Bug introduced in 0d62dd6513 (1.8.3), this restores the original behavior of
always treating SARAWBODY content as raw.