Log the agent conversation through ILogger

[darinkes]

Summary

Wire-level logging for the agent conversation, done the ILogger way: the library logs the data, the attached logger owns the representation.

• SshAgent.Logger (Microsoft.Extensions.Logging.Abstractions, default NullLogger.Instance): at Trace level, every raw protocol message is logged with EventId(1, "AgentMessage").
• SshAgentTraceMessage is the structured log state: it carries Direction and the complete framed Data, and implements IReadOnlyList<KeyValuePair<string, object?>> with Direction/DataLength/Data/{OriginalFormat} properties. Consequences:
  • plain text sinks render only the safe default — Request agent message, 128 bytes — no accidental byte dumps in logs,
  • structured sinks (Serilog, OTel, …) see the raw byte[] as a property and apply their own destructuring/redaction,
  • a custom ILogger pattern-matches the state and renders however it wants — the sample's --verbose does exactly that with a ~25-line HexDumpLogger.
• The XML docs warn that add-identity requests contain private key material; Trace is off in every default logging configuration.
• Dependency note: Microsoft.Extensions.Logging.Abstractions 8.0.3 is the exact version SSH.NET 2025.1.0 already depends on, so consumers on newer SSH.NET pay nothing extra; it is netstandard2.0 and works on all four target frameworks.
• Plumbing: both send paths now share buffered Serialize/ReadMessage/Parse helpers — as a side effect the sync path gains the message-length validation the async path already had.

Sample output of SshNet.Agent.Sample --pageant --verbose:

-> 5 bytes
   0000  00 00 00 01 0b                                   .....
<- 9 bytes
   0000  00 00 00 05 0c 00 00 00 00                       .........
Adding the sample keys ...
-> 128 bytes
   0000  00 00 00 7c 11 00 00 00 0b 73 73 68 2d 65 64 32  ...|.....ssh-ed2
   0010  35 35 31 39 00 00 00 20 4e 56 e8 17 0d 9c cb 8d  5519... NV......

Stacked on #37 (the sample's --verbose builds on the System.CommandLine parser): merge #37 first; GitHub retargets this PR to main automatically when the branch is deleted.

Test plan

• New tests: sync and async sends log exactly two entries with the exact framed bytes, correct level/event id and the summary text; a disabled logger sees nothing (and costs nothing); the structured properties enumerate as documented
• Full suite passes (46 tests); builds on all four target frameworks with CS1591-as-error
• Ran live against a real Pageant with --verbose — hex dump above, including through the Pageant WM_COPYDATA transport