[Rust] Track mock server acks per-stream instead of per-table

[danilonajkov-db]

What changes are proposed in this pull request?

The gRPC mock server previously keyed responses and response indices by table_name, meaning all connections to
the same table shared ack state. In the real server, acks are per-stream — each connection gets its own
independent stream with its own responses.

I need this for multiplexing stream testing.

Changes

mock_grpc.rs

• Responses now keyed by (table_name, stream_id) instead of just table_name
• inject_responses auto-splits at CreateStream boundaries so each stream gets its own response list
• Per-table stream_order queue tracks which stream a new connection should use
• Errors before any CreateStream become per-connection creation errors
• Tests that don't specify CreateStream get a default auto-generated stream
• Same inject_responses(TABLE_NAME, vec![...]) API — no test changes needed

rust_tests.rs

• Updated test_recovery_after_server_unresponsiveness assertion: the "unresponsive" stream now correctly has no
  ack responses (previously it accidentally acked due to a CreateStream being silently consumed as a no-op
  during record processing)

How is this tested?

existing tests