Clarify client handling of embedded binary tool resources

[luisangelrod]

Summary

• document that binary embedded resources are sent as MCP resource contents without SDK-side format conversion
• clarify that rendering and model availability depend on the client, with a text fallback suggested for unsupported formats
• add a protocol serialization regression for an embedded PDF resource
• add a server-path regression that verifies the complete JSON-RPC response and round-trip

Findings

The SDK preserves the PDF MIME type and emits the MCP binary-resource shape:

{
  "result": {
    "content": [
      {
        "type": "resource",
        "resource": {
          "uri": "file:///mypdf.pdf",
          "mimeType": "application/pdf",
          "blob": "JVBERi0xLjcK"
        }
      }
    ]
  },
  "id": 55,
  "jsonrpc": "2.0"
}

This indicates that the PDF-to-image behavior reported in #1261 occurs after the SDK emits the tool result.

Validation

• dotnet build --no-restore — 0 warnings, 0 errors
• protocol PDF round-trip test — passed on net472, net8.0, net9.0, and net10.0
• server wire-response test — passed on net8.0, net9.0, and net10.0
  • net472 is skipped by the existing Windows guard for McpServerTests (Running ModelContextProtocol.Tests in net472 is causing VS test platform to hang #587)

Addresses #1261.