check field length against remaining buffer in mysql Field::Parse

[ubeddulla]

What problem does this PR solve?

Issue Number: resolve N/A

Problem Summary:

The text-protocol MysqlReply::Field::Parse in src/brpc/policy/mysql/mysql_reply.cpp reads a length-encoded field length from the server and calls buf.cutn(&str, len) without checking len against the remaining buffer. IOBuf::cutn clamps to what is available, so a field whose length-encoded prefix claims more bytes than remain makes the string branch allocate len arena bytes, copy only the fewer bytes actually present, and then publish _data.str as a StringPiece of len bytes. The tail is uninitialized arena memory that gets read back by the caller, and the short read desyncs the packet stream. A malicious or compromised MySQL server can trigger it through any text result set. Every sibling length-encoded read in the same file (the binary Field::Parse, Column::Parse, the auth-plugin path, ParseBinaryTime/ParseBinaryDataTime) already rejects len > buf.size(); only this text overload was missing the check.

What is changed and the side effects?

Changed:

Added the same len > buf.size() guard before the cutn in the text Field::Parse, returning PARSE_ERROR_ABSOLUTELY_WRONG on an oversized length. Valid result sets are unaffected since a well-formed field always fits in the buffer.

Side effects:

• Performance effects: none, a single size comparison per field.

• Breaking backward compatibility: none.

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Check List:

• Please make sure your changes are compilable.
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