Fix chi task56 combined.patch: Allow header duplication bug

[AlienKevin]

Summary

The combined.patch for go_chi_task/task56 has two bugs that cause TestMethodNotAllowed and TestSetAllowHeader to always fail, making the oracle solution score 0 for any feature pair involving feature 1.

Bug 1: Middleware reuses routing context across Match() calls

The SetAllowHeader middleware calls rctx.Routes.Match(rctx, method, url) in a loop for each HTTP method, reusing the same rctx. Each Match() call triggers findRoute(), which appends to rctx.methodsAllowed without resetting. After 9 iterations, methodsAllowed contains many duplicate [GET, HEAD] entries, causing the Allow header to contain values like:

```
GET, HEAD, GET, HEAD, GET, HEAD, HEAD, GET, GET, HEAD, GET, HEAD, GET, HEAD, HEAD, GET, HEAD, GET
```

Fix: Use chi.NewRouteContext() for each Match() call to prevent cross-call state pollution.

Bug 2: Core handler uses Set() instead of Add() for Allow header

The MethodNotAllowedHandler builds a comma-joined string and calls w.Header().Set("Allow", "GET, HEAD"), creating a single header value. The tests expect separate values via Header.Values("Allow") — i.e., ["GET", "HEAD"] (2 elements) not ["GET, HEAD"] (1 element).

Fix: Use w.Header().Add("Allow", name) per method to emit individual header values matching the test expectations and RFC conventions.

Verification

Built the chi:task56 Docker image locally from the Dockerfile and ran all tests.

Buggy (original) combined.patch — feature 1 tests fail:
```
--- FAIL: TestMethodNotAllowed/Unregistered_Method (0.00s)
mux_test.go:426: Allow header should contain 2 headers: GET, HEAD. Received: [GET, HEAD]
--- FAIL: TestSetAllowHeader/Unregistered_Method (0.00s)
allowed_methods_test.go:58: GET, HEAD, GET, HEAD, GET, HEAD, GET, HEAD, GET, HEAD, HEAD, GET, GET, HEAD, GET, HEAD, GET, HEAD
```

Fixed combined.patch — all feature tests pass:
```
Feature 1: ok github.com/go-chi/chi/v5, ok github.com/go-chi/chi/v5/middleware
Feature 2: ok github.com/go-chi/chi/v5, ok github.com/go-chi/chi/v5/middleware
Feature 3: ok github.com/go-chi/chi/v5, ok github.com/go-chi/chi/v5/middleware
Feature 4: ok github.com/go-chi/chi/v5, ok github.com/go-chi/chi/v5/middleware
Feature 5: ok github.com/go-chi/chi/v5, ok github.com/go-chi/chi/v5/middleware
```

Test plan

• Verify `TestMethodNotAllowed` passes with the fixed combined.patch
• Verify `TestSetAllowHeader` passes with the fixed combined.patch
• Verify existing passing feature pairs for task56 are unaffected (all 5/5 pass)

[akhatua2]

Thanks Kevin! This looks great to me.