feat: Add MCP Client implementation

[avrabe]

Summary

Add a full MCP client implementation to enable connecting TO MCP servers, not just serving as one.

Motivation

The official rmcp SDK provides complete client-side capabilities. Adding this to PulseEngine would:

• Enable building MCP proxies and gateways
• Allow programmatic testing of MCP servers
• Support multi-hop MCP architectures
• Make PulseEngine a complete SDK (client + server)

Proposed Implementation

Reuse existing code where possible:

Component	Reuse From	Notes
Protocol models	mcp-protocol	All request/response types already defined
Transports	mcp-transport	stdio, HTTP, WebSocket, streamable HTTP
JSON-RPC handling	mcp-server	Message parsing, error handling
Auth	mcp-auth	OAuth client flow, bearer tokens

New Code Needed

// Example API design
let client = McpClient::connect(StdioTransport::new(child_process)).await?;

// Initialize
let server_info = client.initialize(client_info).await?;

// Use server capabilities
let tools = client.list_tools().await?;
let result = client.call_tool("tool_name", json!({"arg": "value"})).await?;

let prompts = client.list_prompts().await?;
let prompt = client.get_prompt("prompt_name", args).await?;

let resources = client.list_resources().await?;
let content = client.read_resource("resource://uri").await?;

// Notifications
client.notify_cancelled(request_id, reason).await?;
client.notify_progress(token, progress, total).await?;

Crate Structure Options

1. Add to mcp-server - Rename to mcp-core or mcp-runtime
2. New mcp-client crate - Separate concerns, optional dependency

Acceptance Criteria

• Connect to MCP servers via stdio transport
• Full protocol support (tools, prompts, resources, completions)
• Progress and cancellation notifications
• Pagination helpers (list_all_* convenience methods)
• Integration with existing auth infrastructure
• Example: programmatic server testing
• Example: MCP proxy/gateway

References

• rmcp client: https://github.com/modelcontextprotocol/rust-sdk/blob/main/crates/rmcp/src/service/client.rs