refactor: improve bidirectional capability handling and error reporting

[tzolov]

Major changes:

• Add capability check methods (rootsEnabled, elicitEnabled, sampleEnabled) to both sync and async request contexts
• Replace Optional return types with direct returns and throw IllegalStateException when capabilities not supported
• Change from warning logs to exceptions for unsupported operations in stateless contexts
• Removed stateless context implementations (StatelessAsyncRequestContext, StatelessMcpSyncRequestContext)
• Add bidirectional parameter filtering for stateless providers (tools, resources, prompts, completions)
• Update tests to verify new exception-based error handling
• Updated README

Breaking changes:

• McpSyncRequestContext methods now return direct types instead of Optional
• McpAsyncRequestContext methods now return Mono.error() instead of Mono.empty() for unsupported operations
• Stateless context methods throw IllegalStateException instead of logging warnings

This improves API clarity by making capability checks explicit and error handling more predictable.