Fix OP_MSG auth_source handling

[haoxianhan]

Summary

Fix OP_MSG authentication when auth_source differs from database.

The auth request already carried the correct database, but the worker sent OP_MSG commands using the connection default database instead. This caused auth failures when auth_source and database were different.

Add regression coverage for both cases and run it in the MongoDB 8 CI job.

Testing

• rebar3 ct --suite=test/op_msg_auth_source_SUITE.erl

[haoxianhan]

A minimal repro

docker run -d --name mongo-auth-repro \
  -e MONGO_INITDB_ROOT_USERNAME=admin \
  -e MONGO_INITDB_ROOT_PASSWORD=admin \
  -p 37118:27017 \
  mongo:8.2.5

pre-fix revision (3e9e372):

  mc_worker_api:connect([
    {database, <<"admin">>},
    {auth_source, <<"admin">>},
    {login, <<"admin">>},
    {password, <<"admin">>},
    {host, "localhost"},
    {port, 37118},
    {use_legacy_protocol, false}
  ]).

work fine, returns:

  {ok, Connection}

but changing only the database field to <<"test">>:

  mc_worker_api:connect([
    {database, <<"test">>},
    {auth_source, <<"admin">>},
    {login, <<"admin">>},
    {password, <<"admin">>},
    {host, "localhost"},
    {port, 37118},
    {use_legacy_protocol, false}
  ]).

fails with:

 {'EXIT',
   {<<"Can't pass authentication">>,
    [{mc_auth_logic,scram_sha_1_auth,4,
      [{file,".../mc_auth_logic.erl"},{line,61}]},
     {mc_worker_api,connect,1,
      [{file,".../mc_worker_api.erl"},{line,73}]} ...

With this PR (2694607), database = <<"test">>, auth_source = <<"admin">> returns:

  {ok, Connection}

[comtihon]

Thank you!