Coverage: missing tests (protocol)

* compute_column_type json (MariaDB specific?)
* deserialize_execute_response marks some error codes as not hit
* deserialize_text_ymd: separator
* deserialize_text_value_date: excess characters
* deserialize_text_value_datetime: separator
* deserialize_text_value_time: separator
* deserialize_text_value_float: res.ptr != end
* deserialize_binary_field_string: error
* should_perform_full_auth: branch
* next_char_utf8mb4: not enough size checks

See if we could write some of these in a more coverage-friendly way.

Locate the named protocol entry points, including compute_column_type, deserialize_execute_response, the deserialize_text_value variants, deserialize_binary_field_string, should_perform_full_auth, and next_char_utf8mb4. Start by running the existing coverage suite and inspecting nearby tests. Add focused tests for the listed uncovered branches and confirm the resulting coverage improvements.

Written by the indexing model from the issue text.