Improve MCP Client Validation and Protocol Compliance

[tzolov]

Please do a quick search on GitHub issues first, the feature you are about to request might have already been requested.

Expected Behavior

The MCP client should:

1. Properly validate its state and server capabilities before operations
2. Only expose client-appropriate methods
3. Handle protocol version negotiation clearly

Example of expected validation:

// Client should validate state before operations
public Mono<McpSchema.CallToolResult> callTool(McpSchema.CallToolRequest request) {
    if (!this.isInitialized()) {
        return Mono.error(new McpError("Client must be initialized before calling tools"));
    }
    if (this.serverCapabilities.tools() == null) {
        return Mono.error(new McpError("Server does not provide tools capability"));
    }
    return this.mcpSession.sendRequest(...);
}

Current Behavior

1. Client operations don't validate initialization state or server capabilities, potentially leading to runtime errors
2. Client exposes server-side notification methods that shouldn't be called by clients:

// These methods should not exist in client
public Mono<Void> sendResourcesListChanged()
public Mono<Void> promptListChangedNotification()

3. Protocol version handling lacks clear documentation and validation

Context

This issue affects developers using the MCP client in several ways:

• Lack of validation can lead to confusing runtime errors when client isn't properly initialized
• Exposed server methods can lead to protocol violations if called by client applications
• Unclear protocol version handling can cause compatibility issues

Current workaround:

• Developers must manually check initialization state and capabilities before operations, but this is error-prone and leads to duplicated code.