Read message type from fix string without regex

[jkulubya]

I'm investigating how to have the message type as part of the log's context when a message is sent/received in #899, and that might require calling GetMsgType on the hot path. This pr removes the regex from that method, in case that's the way forward.

BenchmarkDotNet v0.14.0, NixOS 24.11 (Vicuna)
    11th Gen Intel Core i7-1165G7 2.80GHz, 1 CPU, 8 logical and 4 physical cores
    .NET SDK 8.0.307
      [Host]     : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
      DefaultJob : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

    | Method | Mean      | Error    | StdDev    | Gen0   | Allocated |
    |------- |----------:|---------:|----------:|-------:|----------:|
    | Manual |  35.33 ns | 2.754 ns |  8.122 ns | 0.0038 |      24 B |
    | Regex  | 322.38 ns | 9.405 ns | 25.265 ns | 0.0648 |     408 B |

Manual is this PR, Regex is current code.