API Gateway emulator builds incomplete APIGatewayProxyRequest; AspNetCoreServer NREs on null Path / RequestContext / headers

### Describe the bug

When invoking an ASP.NET Core Lambda through **Lambda Test Tool v2 API Gateway emulator**, the emulator builds an `APIGatewayProxyRequest` that is incomplete compared with a real API Gateway event:

- `RequestContext` is often **null / missing** (no `Path`, `HttpMethod`, `ApiId`, `Identity`, etc.)
- In some paths, `Path`, `HttpMethod`, or path-parameter / header values can be **null**

`Amazon.Lambda.AspNetCoreServer` (`APIGatewayProxyFunction.MarshallRequest` / path + header utilities) assumes these fields are present and throws **NullReferenceException** (or fails marshalling) instead of degrading safely.

This breaks the local inner loop for ASP.NET Core on Lambda behind the emulator, even when the route **is** a normal Lambda integration (not an HTTP proxy backend).

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

- Emulator-built proxy events should populate a minimal but valid `RequestContext` (and Path / HttpMethod), similar to API Gateway test-invoke.
- `Amazon.Lambda.AspNetCoreServer` should null-guard `Path`, path parameters, and header values so incomplete events fail clearly or use safe defaults, not NRE.

### Current Behavior

Marshalling crashes or misbehaves when the emulator omits `RequestContext` / leaves path or header values null.

### Reproduction Steps

1. Run Lambda Test Tool v2 with API Gateway emulator enabled.
2. Configure a route to an ASP.NET Core function using `Amazon.Lambda.AspNetCoreServer` (`APIGatewayProxyFunction` / Hosting).
3. Call the route via the emulator HTTP URL (REST or HTTP API mode).
4. Observe NRE / marshalling failure when `RequestContext` or `Path` is null on the synthesized event.

Minimal observation: deserialize/inspect the event the emulator sends to the Runtime API and compare with a real API Gateway proxy event — `RequestContext` is missing on the emulator path.

### Possible Solution

1. **Test Tool emulator:** always set `APIGatewayProxyRequest.RequestContext` (and Path / HttpMethod) when translating HTTP → Lambda event.
2. **AspNetCoreServer:** null-safe marshalling for `Path`, `HttpMethod`, path parameters, and multi/single-value headers.

We have a working fix on a fork (happy to open a PR once this issue exists).

### Additional Information/Context

- Packages: `Amazon.Lambda.AspNetCoreServer`, `Amazon.Lambda.TestTool` (v2 API Gateway emulator)
- Related (separate feature request): HTTP proxy integration so non-Lambda backends share the emulator origin — **not** this bug.

### AWS .NET SDK and/or Package version used

Amazon.Lambda.AspNetCoreServer 10.2.1
Amazon.Lambda.AspNetCoreServer.Hosting 2.2.1
Amazon.Lambda.TestTool 0.15.1
Amazon.Lambda.Core 3.3.0
Amazon.Lambda.APIGatewayEvents 3.0.1

### Targeted .NET Platform

.NET 10, AspNetCoreServer 10.2.1

### Operating System and version

Windows 11

Inizia esaminando la traduzione degli eventi HTTP-to-Lambda dell’emulatore API Gateway di Lambda Test Tool v2 e confronta il suo evento con un vero evento proxy di API Gateway. Poi traccia APIGatewayProxyFunction.MarshallRequest di Amazon.Lambda.AspNetCoreServer e le relative utilità per percorsi e header. Il lavoro è completato quando gli eventi incompleti dell’emulatore non causano più NRE né errori di marshalling, con una gestione sicura dei campi null elencati.

Scritto dal modello di indicizzazione a partire dal testo della issue.