Policy: GetAttributesByValueFqns RPC request validation should happen in protovalidate rules

[ryanulit]

Background

Originally brought up in a separate PR: https://github.com/opentdf/platform/pull/1633/files/e34689983f9be1b52af49254f5ccfa7eb8baf5f9#r1801590942

The request validation checks at the beginning of the RPC method should be moved to protovalidate rules. Additionally, see comments below for other changes.

Acceptance Criteria

• remove r.GetWithValue() nil check and any existing protovalidate rules requiring it in the request, then add an optional comment on the request proto field
• move fqns array length check to protovalidate with a min and max items range of 1-250 (sensible default for now)
• unit test coverage for new rules

[ryanulit]

After further discussion with @jakedoublev and @jrschumacher, we've decided the FQN /value/ format validation is no longer necessary and can be removed since the request will return Not Found if any of the FQNs are invalid. Additionally, r.GetWithValue() is not even used anywhere in the GetAttributesByValueFqns DB layer function except for required request validation, so this will be made optional now but remain on the request proto for future use. The description will be updated accordingly.