Add bidirectional communication to local server

[Copilot]

Extended bidirectional communication (sendNotification, sendRequest) from relay-only to both relay and local server implementations.

Changes

Local Server (mcp_server.lua)

• Added sendNotification() and sendRequest() methods with message queue and callback tracking
• Added /poll endpoint for long-polling (returns 204 when empty, 200 with message)
• Added /response endpoint for client responses to server-initiated requests
• Initialized instance-specific tables in constructor (following relay pattern)

Protocol Layer (mcp_protocol.lua)

• Added transport abstraction via setTransport() to support both MCPServer and MCPRelay
• Added notifyResourcesUpdated() to send resource update notifications through transport
• Added checkAndNotifyResourceChanges() for automatic change detection and notification

Integration (main.lua)

• Set transport on protocol when starting local or relay server
• Added scheduleResourceChangeCheck() with 0.5s polling interval
• Proper cleanup of resource check task on server stop

Usage

Both local and relay servers now support server→client communication:

-- Resource changes automatically trigger notifications
protocol:checkAndNotifyResourceChanges()  
-- Sends: {"jsonrpc":"2.0","method":"notifications/resources/updated","params":{"uri":"book://current/context"}}

-- Manual notifications
transport:sendNotification({
    jsonrpc = "2.0",
    method = "notifications/message",
    params = {level = "info", message = "Page changed"}
})

-- Bidirectional requests
transport:sendRequest({
    jsonrpc = "2.0",
    id = 1,
    method = "sampling/createMessage",
    params = {...}
}, function(response) 
    -- Handle response
end)

Enables MCP features requiring server→client communication: resource update notifications, logging, sampling requests, and elicitation in both deployment modes.

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