Refactor McpFunctionMetadataTransformer using Builder pattern with pluggable steps

[liliankasem]

Summary

Decomposes the monolithic McpFunctionMetadataTransformer into a fluent McpBindingBuilder with pluggable extension method steps. Each transformation concern lives in its own file and can be added or removed without modifying the builder itself.

Motivation

The transformer had grown to ~210 lines mixing parsing, tool property resolution, metadata enrichment, and property binding patching in a single class. This refactor improves:

• Separation of concerns — each step is a self-contained extension method
• Extensibility — new steps are added by creating a new file, no existing code changes
• Testability — each step can be tested in isolation

Changes

New: McpBindingBuilder/

File	Purpose
McpBindingBuilder.cs	Fluent builder core — HasBindings, Build(), binding parsing. Never needs to change.
McpBuilderContext.cs	Shared state between steps (bindings, function metadata, resolved properties)
McpParsedBinding.cs	Typed representation of a parsed MCP binding
MetadataMerger.cs	Static utility for merging fluent + attributed metadata JSON

New: McpBindingBuilder/Steps/

File	Purpose
AddToolPropertiesExtension.cs	Resolves tool properties (configured options → attribute reflection fallback)
AddMetadataExtension.cs	Applies fluent API + [McpMetadata] attribute metadata onto trigger bindings
PatchPropertyBindingsExtension.cs	Patches mcpToolProperty bindings with type info from resolved properties

Modified: McpFunctionMetadataTransformer.cs

Reduced from ~210 lines to ~40 lines. Now a thin loop:

new McpBindingBuilder(function, logger)
    .AddToolProperties(toolOptionsMonitor)
    .AddMetadata(toolOptionsMonitor, resourceOptionsMonitor)
    .PatchPropertyBindings()
    .Build();

How to add a new step

Create a single file with an extension method — no other files change:

// Steps/AddMyNewStepExtension.cs
internal static class AddMyNewStepExtension
{
    public static McpBindingBuilder AddMyNewStep(this McpBindingBuilder builder, ...)
    {
        var context = builder.Context;
        // transform bindings...
        return builder;
    }
}