Add public constructors to McpClient types for reusing cached definitions

[Copilot]

Enables caching tool, prompt, resource, and resource template definitions to avoid redundant List*Async calls on reconnect scenarios.

Changes

API Additions

• Added public McpClientTool(McpClient client, Tool tool, JsonSerializerOptions? serializerOptions = null) constructor
• Added public McpClientPrompt(McpClient client, Prompt prompt) constructor
• Added public McpClientResource(McpClient client, Resource resource) constructor
• Added public McpClientResourceTemplate(McpClient client, ResourceTemplate resourceTemplate) constructor
• All constructors validate non-null parameters
• Tool constructor defaults serializer options to McpJsonUtilities.DefaultOptions

Tests

• Added 12 test cases for McpClientTool covering constructor validation, tool reuse across client instances, manually constructed Tool objects, and AIFunction integration
• Added 6 test cases for McpClientPrompt covering constructor validation, prompt reuse, and metadata preservation
• Added 6 test cases for McpClientResource covering constructor validation, resource reuse, and metadata preservation
• Added 5 test cases for McpClientResourceTemplate covering constructor validation, template reuse, and metadata preservation

Usage

// Cache tool definition from initial connection
var tools = await client1.ListToolsAsync();
var toolDef = tools[0].ProtocolTool;

// Reuse on reconnect without ListToolsAsync round-trip
var client2 = await McpClient.CreateAsync(transport2);
var tool = new McpClientTool(client2, toolDef);
var result = await tool.CallAsync(args);

Similar patterns work for prompts, resources, and resource templates using their respective constructors and protocol definitions.

• Fixes Reuse McpClientTool across different McpClient instances #934

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Reuse McpClientTool across different McpClient instances</issue_title>
<issue_description>Sometimes it makes sense to cache tool definitions and not do a ListToolsAsync on every reconnect.

Adding a WithMcpClient() that creates a new instance with the provided client to McpClientTool would enable this pattern.
</issue_description>

<agent_instructions>Add a public ctor to McpClientTool that accepts McpClient client, Tool tool, JsonSerializerOptions? serializerOptions = null, defaulting the serializer options to McpJsonUtilities.DefaultOptions if none is specified. Add tests that validate resulting McpClientTool instances can be used to successfully invoke manually constructed Tool instances representing a tool available on a particular McpClient.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Reuse McpClientTool across different McpClient instances #934

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[stephentoub]

@copilot, please add a similar ctor (and associated tests) for McpClientPrompt, McpClientResource, and McpClientResourceTemplate. When there's already a relevant test file (as there is for McpClientResourceTemplate, add to the existing file rather than creating a new one).

[Copilot]

@copilot, please add a similar ctor (and associated tests) for McpClientPrompt, McpClientResource, and McpClientResourceTemplate. When there's already a relevant test file (as there is for McpClientRe...

Added public constructors and tests for McpClientPrompt, McpClientResource, and McpClientResourceTemplate in commit 401e5da. All 29 tests are passing.

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