Add custom tool result marshaling

[andyst-dev]

Summary

• add a MarshalResult delegate to McpServerToolCreateOptions
• pass the custom delegate to AIFunctionFactoryOptions while preserving the existing pass-through default
• preserve the delegate when deriving tool options
• add a regression test covering the result value, declared return type, cancellation token, and transformed tool response

Motivation and context

This lets reflection-created MCP tools customize AIFunction result marshaling without recreating the SDK's private option-derivation logic.

Closes #1602.

Validation

• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --framework net10.0 --filter FullyQualifiedName~SupportsCustomResultMarshaling — 1 passed
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --framework net10.0 --filter "Execution!=Manual" — 2,335 passed, 3 skipped
• make generate-docs — Release build and DocFX succeeded with 0 warnings and 0 errors
• git diff --check

Breaking changes

None. The new option is nullable and the existing pass-through behavior remains the default.