Quote procname and user variables with backticks in callproc

[Copilot]

callproc passed the procedure name and user variable names unquoted into SQL, making it unsafe for names containing reserved words or special characters.

Changes

• _backquote_escape(s) — new helper that doubles backticks in a bytes value to safely embed it as a backtick-quoted identifier
• callproc — escapes procname and wraps it in backticks in both the CALL statement and the @_procname_n`` user variables used for args/OUT params

-- before
SET @_my_proc_0=...; CALL my_proc(@_my_proc_0)

-- after
SET @`_my_proc_0`=...; CALL `my_proc`(@`_my_proc_0`)

Ports PyMySQL/PyMySQL#1225.