fix: make PCNTL extension optional for StdioServerTransport

Signal handling now gracefully degrades when PCNTL extension is unavailable or the event loop doesn't support signals. The transport continues to work normally since termination also happens when STDIN closes.

test: Add tests for various handler types in RegisteredElement

fix(tests): wrong enum used in DispatcherTest

feat: add backward compatibility for CompletionProvider providerClass…

… parameter

Maintains support for existing `providerClass` parameter while encouraging
migration to the new `provider` parameter for consistency.

fix(tests): update expectations for session in initialize

refactor: make instructions parameter nullable in ServerBuilder

fix: improve error handling in ServerBuilder manual registration

This fixes the poor error reporting where users would only see generic
"X error(s) occurred during manual registration" instead of the actual
underlying error that caused the registration failure.

fix: update validation error keyword for empty data object against sc…

…hema

docs: comprehensive README update for 2025-03-26 protocol

Update documentation to reflect current architecture with proper schema generation,
deployment strategies, and corrected configuration examples.

fix: Disable registry notifications during manual registrations to pr…

…event unwanted triggers