Remove redundant CanonicalizeHeaderKey from metadata iteration paths

Problem:
metadataToTransportRequest and getApplicationHeaders both call
transport.CanonicalizeHeaderKey (strings.ToLower) on every key while
iterating gRPC metadata.MD. These keys are already guaranteed to be
lowercase, making the calls redundant CPU work on every inbound and
response path.

Fix:
Remove the CanonicalizeHeaderKey call from both functions. Also
normalize a test case in TestGetApplicationHeaders that used a
mixed-case metadata key via a raw metadata.MD literal, which no
longer reflects a realistic scenario.

Safety — three independent guarantees that metadata keys are lowercase:

1. HTTP/2 spec (RFC 7540 s8.1.2): header field names MUST be
   converted to lowercase prior to their encoding in HTTP/2.
   The requirement is on the sender; receivers are guaranteed
   to see lowercase keys on the wire.
   https://httpwg.org/specs/rfc7540.html#rfc.section.8.1.2

2. grpc-go PR #4416 (merged v1.39, June 2021): FromIncomingContext
   now explicitly lowercases all keys, including those injected by
   interceptors.
   grpc/grpc-go#4416

3. yarpc-go uses grpc-go v1.67.3 (go.mod), well past v1.39.

Call sites:
- metadataToTransportRequest: called from handler.go via
  metadata.FromIncomingContext (server inbound path)
- getApplicationHeaders: called from outbound.go with response
  trailer metadata (client response path)

Impact:
Eliminates one strings.ToLower scan per metadata entry on both the
server inbound and client response paths.

Made-with: Cursor

Author: kshitijsuri90

transport/grpc/headers.go

@@ -147,8 +147,7 @@ func metadataToTransportRequest(md metadata.MD) (*transport.Request, error) {
 		default:
 			return nil, yarpcerrors.InvalidArgumentErrorf("header has more than one value: %s:%v", header, values)
 		}
-		header = transport.CanonicalizeHeaderKey(header)
-		// skip routing header
+		// gRPC metadata keys are already lowercase.
 		if routingHeaders[header] {
 			continue
 		}
@@ -223,7 +222,7 @@ func getApplicationHeaders(md metadata.MD) (transport.Headers, error) {
 	}
 	headers := transport.NewHeadersWithCapacity(md.Len())
 	for header, values := range md {
-		header = transport.CanonicalizeHeaderKey(header)
+		// gRPC metadata keys are already lowercase.
 		if isReserved(header) {
 			continue
 		}

transport/grpc/headers_test.go

@@ -253,7 +253,7 @@ func TestGetApplicationHeaders(t *testing.T) {
 				"rpc-service":         []string{"foo"}, // reserved header
 				"test-header-empty":   []string{},      // no value
 				"test-header-valid-1": []string{"test-value-1"},
-				"test-Header-Valid-2": []string{"test-value-2"},
+				"test-header-valid-2": []string{"test-value-2"},
 			},
 			wantHeaders: map[string]string{
 				"test-header-valid-1": "test-value-1",