Add BindRawQueryParameter which operates on the raw (undecoded) query
string instead of pre-parsed url.Values. For form/explode=false
parameters, this splits on literal commas before URL-decoding each part,
correctly preserving %2C as a literal comma in values.

Also adds findRawQueryParam internal helper, comprehensive tests, and
round-trip (serialize/deserialize) tests.

Deprecates BindQueryParameter, which cannot distinguish delimiter commas
from literal commas because url.Values pre-decodes %2C.

Fixes oapi-codegen#91

Note: oapi-codegen/oapi-codegen will need to update its generated code
to call BindRawQueryParameter (passing r.URL.RawQuery) once this change
is released.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>