Update content according to latest spec

[stephentoub]

Closes #508
Closes #517
Closes #518

[stephentoub]

@halter73, @eiriktsarpalis, I've updated this with the most recent updates to the spec.

[Copilot]

Pull Request Overview

This pull request updates the SDK to conform to the latest protocol specification changes. Key updates include renaming response and notification types (e.g., CallToolResponse to CallToolResult, CancelledNotification to CancelledNotificationParams) and adding new properties such as LastModified in annotations.

Reviewed Changes

Copilot reviewed 106 out of 106 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/ModelContextProtocol.Core/Protocol/CallToolRequestParams.cs	Updated documentation references to the new CallToolResult type.
src/ModelContextProtocol.Core/Protocol/Annotations.cs	Added a new LastModified property with ISO 8601 guidelines.
src/ModelContextProtocol.Core/McpSession.cs	Adjusted usage of cancellation and notification parameter types.
src/ModelContextProtocol.Core/McpJsonUtilities.cs	Updated JSON serialization context types to reflect spec changes.
src/ModelContextProtocol.Core/Client/McpClientTool.cs	Changed method signatures and usages from CallToolResponse to CallToolResult and added a Title property.
src/ModelContextProtocol.Core/Client/McpClientExtensions.cs	Refactored LINQ expressions and adjusted parameter types in line with the new spec.
src/ModelContextProtocol.Core/Client/McpClient.cs	Made similar type updates and improved handling of notifications.
src/ModelContextProtocol.Core/AIContentExtensions.cs	Updated conversion methods from Content to ContentBlock and applied new switch expressions.
samples/*	Updated sample tools and program logic to reference the new types and properties.

Comments suppressed due to low confidence (2)

samples/TestServerWithHosting/Tools/SampleLlmTool.cs:23

• [nitpick] Consider adding a fallback value (e.g., using the null-coalescing operator) to ensure a non-null output in case the cast to TextContentBlock fails.

return "LLM sampling result: " + (sampleResult.Content as TextContentBlock)?.Text;

samples/EverythingServer/Tools/SampleLlmTool.cs:20

• [nitpick] Consider adding a fallback value (e.g., using the null-coalescing operator) to ensure a non-null string is returned if the cast to TextContentBlock fails.

return "LLM sampling result: " + (sampleResult.Content as TextContentBlock)?.Text;