fix: registerCapabilities registers logging/setLevel handler

[giulio-leone]

Problem

When logging capability is added via registerCapabilities() instead of the constructor options, the logging/setLevel request handler is not registered. This causes clients to receive a 'Method not found' error when calling setLoggingLevel().

// This works ✅
let a = new Server({}, { capabilities: { logging: {} } });

// This doesn't ✅ → ❌
let b = new Server();
b.registerCapabilities({ logging: {} });
// b has no logging/setLevel handler

Root Cause

The logging/setLevel handler is only registered in the constructor when this._capabilities.logging is truthy. When registerCapabilities() is called later to add logging, it only merges capabilities but doesn't register the handler.

Fix

Extract the logging handler setup into a private _registerLoggingHandler() method and call it from both:

1. The constructor (when logging is in initial capabilities)
2. registerCapabilities() (when logging capability is added after construction, and wasn't there before)

Tests

Two new integration tests:

• registerCapabilities should register logging/setLevel handler: Creates a Server without logging, adds it via registerCapabilities, then verifies the client can successfully call setLoggingLevel
• logging in constructor capabilities should register logging/setLevel handler: Verifies the existing constructor path still works

All 86 server integration tests pass (2 consecutive clean runs).

Fixes #1464

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commit: 2de4c27

[giulio-leone]

Friendly ping — CI is green and this is ready for review. Happy to address any feedback. Thanks!