Support specifying OutputSchema type independently of return type for tools returning CallToolResult

[jcauv4]

Is your feature request related to a problem? Please describe.
When a tool method returns CallToolResult directly (to control properties like Meta, IsError, or manually populate StructuredContent), the SDK cannot generate a meaningful OutputSchema for the tool. The output schema either reflects the shape of CallToolResult itself—which is an internal protocol type and useless to clients—or is omitted entirely.

This forces developers to choose between:

• Returning a strongly-typed object to get a proper OutputSchema, but losing control over Meta, IsError, and other CallToolResult properties.
• Returning CallToolResult for full response control, but with no way to advertise the actual shape of StructuredContent to clients.

Describe the solution you'd like
Allow developers to specify the output schema type independently of the return type, for example via an attribute parameter or a create option:

// Option A: Attribute-based
[McpServerTool(UseStructuredContent = true, OutputSchemaType = typeof(MyResponseType))]
public static CallToolResult my_tool(string input) { ... }



// Option B: Via McpServerToolCreateOptions
McpServerToolCreateOptions options = new()
{
    UseStructuredContent = true,
    OutputSchemaType = typeof(MyResponseType),
};

When OutputSchemaType is set, the SDK would generate the OutputSchema from that type while still allowing the method to return CallToolResult with full control over the response.

Describe alternatives you've considered

• Returning a typed object: Works for simple cases, but precludes setting Meta, IsError, or customizing Content independently of StructuredContent.
• Schema node transformation workaround: Creating a stub tool with McpServerTool.Create(() => new T()) solely to extract its output schema, then using SchemaCreateOptions.TransformSchemaNode on the real tool to replace the CallToolResult schema with the stub's. This works but is fragile, non-obvious, and requires significant boilerplate.
• Manually setting StructuredContent without UseStructuredContent: Provides structured data to clients but no OutputSchema is advertised in tools/list, so clients can't validate or understand the response shape ahead of time.

Additional context
This gap is particularly impactful in scenarios where tools need to return both structured content and out-of-band metadata via CallToolResult.Meta. 

[Izelude]

duplicate:
#930