Add GuardHandler for basic request filtering and DI extensions

[carldebilly]

• Introduced GuardHandler for lightweight filtering of requests with configurable constraints
• Added DI-friendly extensions to register and auto-wire GuardHandler into the server.

[Copilot]

Pull Request Overview

This PR introduces the GuardHandler for basic request filtering with configurable constraints and adds DI-friendly extensions to simplify handler registration and auto-wiring into the server pipeline.

• Adds GuardHandler with request validation for URL length, headers, body size, HTTP methods, and host filtering
• Implements generic DI extensions for automatic handler registration into server pipeline
• Creates specialized DI extensions for GuardHandler with options support

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Yllibed.HttpServer/Handlers/GuardHandler.cs	Core handler implementing request filtering logic with configurable limits
Yllibed.HttpServer/Extensions/ServiceCollectionExtensions.cs	Generic DI extensions for automatic handler registration with HandlerRegistrationService
Yllibed.HttpServer/Extensions/GuardExtensions.cs	Specialized DI extensions for GuardHandler registration and auto-wiring
Yllibed.HttpServer.Tests/GuardHandlerFixture.cs	Unit tests for GuardHandler validation scenarios
Yllibed.HttpServer.Tests/GuardHandlerDiFixture.cs	Tests for GuardHandler DI registration functionality
Yllibed.HttpServer.Tests/GenericDiHandlersFixture.cs	Tests for generic handler DI registration extensions
README.md	Documentation for GuardHandler usage patterns and DI integration

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