fix(pure_rust): restore Firebird 2.5 support broken by #177

[maratth]

Since #177 (c5ab7c3), attach fails on Firebird 2.5 with:

Invalid clumplet buffer structure: buffer end before end of clumplet - clumplet too long

The DPB builder selected the wide clumplet format (isc_dpb_version2, 4-byte
lengths) whenever every parameter was shorter than 255 bytes — the condition
was inverted, and that format only exists since Firebird 3.0. Firebird 2.5 reads
the DPB with single-byte lengths, so its parser desynchronised immediately. The
other branch was broken too: it emitted version 1 with a length truncated to
u8.

Fixed by selecting the wide format only when a parameter actually exceeds 255
bytes. Every value the driver sends is well under that, so both 2.5 and 3+ now
receive a version 1 DPB, as before #177.

Tested against Firebird 2.5 with the pure_rust backend.