fix(brotli): don't log expected non-standard ETag at error level

[AlinsRan]

TL;DR

brotli.lua logged an expected non-quoted upstream ETag at error level (printing ... : nil), spamming the error log.

Cause

weak_etag_header() downgrades a strong ETag to a weak one (Brotli alters the body, so a strong validator no longer holds). When the upstream ETag is not a quoted string the regex simply does not match — an expected case — but the code logs it at error level with a nil err:

local matched, err = ngx.re.match(etag, regex, "jo")
if not matched or err then
    core.response.set_header("Etag", nil)
    core.log.error("no standard etag or regex match failed: ", err)  -- err is nil here
    return
end

Fix

• Only log at error level when ngx.re.match actually returns an error; otherwise drop the ETag silently (behavior unchanged).
• Tighten the capture group from (.*) to ([^"]*) so a malformed ETag containing internal quotes is treated as non-standard (and dropped) rather than weakened into another malformed value.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change (N/A)
• I have verified that the changes pass the existing tests