Add DP024 analyzer for unregistered handler implementations

[Skymly]

Summary

Add an analyzer (DP024) that reports concrete IHandler<TContext> implementations missing [HandlerOrder] when the compilation already registers handlers for that context type—aligned with DP006 (Strategy) and DP023 (Factory).

Acceptance criteria

• New DiagnosticIds.HandlerOrderUnregisteredImplementation = DP024
• UnregisteredHandlerAnalyzer in DesignPatterns.Analyzers
• Tests in DesignPatterns.Analyzers.Tests (reports DP024; no false positive when [HandlerOrder] present)
• Document in docs/ChainOfResponsibility.md and AnalyzerReleases.Unshipped.md
• No CodeFix in this PR (optional follow-up)

Module

DesignPatterns.Diagnostics/, DesignPatterns.Analyzers/, tests/DesignPatterns.Analyzers.Tests/, docs/