fix(postgrest): don't put a dot before nested and/or groups

[fresh55]

closes #336

three or more || terms nest, and the nested group got a dot, which postgrest rejects:

before  or=(or.(first_name.like.*Ran*,middle_name.like.*Ran*),last_name.like.*Ran*)   400 PGRST100
after   or=(or(first_name.like.*Ran*,middle_name.like.*Ran*),last_name.like.*Ran*)    200

only column filters take the dot, a group is already parenthesized. negated groups nest as not.and(...).

also fixes .Or()/.And() with a nested group, both go through PrepareFilter.

didn't flatten to or=(a,b,c) - that was tried in supabase-community/postgrest-csharp#110 and needs a visitor rewrite. the nested form is valid.