HTTP GET triggers should auto-handle HEAD requests (or return clearer error)

## Summary

When an Azure Function has an HTTP trigger configured for GET only, HEAD requests return a generic 404 from the routing layer rather than being auto-handled or returning a clear "method not allowed" error. This causes significant debugging confusion.

## Problem

A function defined as:

```csharp
[Function("RedirectUrl")]
public async Task RedirectUrl(
[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "r/{shortCode}")] HttpRequest req,
string shortCode)
```

**Behavior:**
- `GET /api/r/abc123` → **302 Found** ✅ (works)
- `HEAD /api/r/abc123` → **404 Not Found** ❌ (confusing)

The 404 comes from the routing layer before the function is invoked, with no indication that HEAD is unsupported.

## Why This Matters

1. **Common testing pattern**: Developers frequently use `curl -I` (HEAD) to test redirects and check response headers without downloading the body
2. **Debugging time waste**: I spent ~20 minutes debugging why redirects "weren't working" before realizing HEAD vs GET was the issue
3. **HTTP spec expectation**: RFC 7231 states "The HEAD method is identical to GET except that the server MUST NOT send a message body" - many developers expect GET endpoints to auto-handle HEAD

## Suggested Fix (in priority order)

### Option A: Auto-handle HEAD for GET triggers (Preferred)
When a function accepts GET, automatically handle HEAD by:
1. Invoking the function with the HEAD request
2. Returning headers only (strip body)

This matches behavior of most web frameworks (ASP.NET Core, Express, Flask, etc.)

### Option B: Return 405 Method Not Allowed
If HEAD is not supported, return:
```
HTTP/1.1 405 Method Not Allowed
Allow: GET, POST
```

This clearly indicates the method isn't supported and which methods are.

### Option C: Better documentation
At minimum, document that HEAD requests require explicit `"head"` in the trigger:
```csharp
[HttpTrigger(AuthorizationLevel.Anonymous, "get", "head", Route = "...")]
```

## Environment

- Azure Functions Flex Consumption
- .NET 10 isolated worker (net10.0)
- Functions Runtime: 4.1046.x

## Reproduction

1. Create function with `[HttpTrigger(..., "get", ...)]`
2. Deploy to Azure
3. `curl -I https://.azurewebsites.net/api/` → 404
4. `curl https://.azurewebsites.net/api/` → 200/302 (works)

The inconsistency between HEAD and GET is unexpected and causes debugging confusion.

The issue names no repository files or tests. Start by reproducing the HttpTrigger GET-only case with curl, comparing GET and HEAD and confirming that the 404 occurs in the routing layer before invocation. Clarify which proposed outcome is accepted—automatic HEAD handling, a 405 response, or documentation—then add coverage or documentation matching that decision.

Written by the indexing model from the issue text.