Fix: Correct return type and preserve response timing info (#1773)

The commit introduces the following enhancements:

1. Type Correction for race-network-and-fetch-event: Modified the race-network-and-fetch-event algorithm to
    consistently return the correct value type. This addresses a previous inconsistency where the algorithm could
    yield a value of a different, incorrect type.
2. Preservation of Response Timing Information: Ensured that timing information within the response, particularly the
    fetch event dispatch time, is retained when the fetch event's value is utilized. This resolves an issue where such
    timing data was previously being discarded.

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3281,9 +3281,11 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
                       1. [=queue/Enqueue=] |raceFetchHandlerResult| to |queue|.
                   1. Wait until |queue| is not empty.
                   1. Let |result| be the result of [=dequeue=] |queue|.
-                  1. Set |timingInfo|'s [=service worker timing info/worker final router source=] be set to |result|'s [=race result/used route=].
-                  1. Set |result|'s [=service worker timing info=] be set to |timingInfo|.
-                  1. Return |result|'s [=/race response=].
+                  1. Let |routedResponse| be |result|'s [=race result/routed response=].
+                  1. If |result|'s [=race result/used route=] is {{RouterSourceEnum/"network"}}, then:
+                      1. Set |routedResponse|'s [=service worker timing info=] be set to |timingInfo|.
+                  1. Set |routedResponse|'s [=service worker timing info=]'s [=service worker timing info/worker final router source=] be set to |result|'s [=race result/used route=].
+                  1. Return |routedResponse|.
               1. Assert: |source| is "{{RouterSourceEnum/fetch-event}}"
       1. If |request| is a [=navigation request=], |registration|'s [=navigation preload enabled flag=] is set, |request|'s [=request/method=] is \`<code>GET</code>\`, |registration|'s [=active worker=]'s [=set of event types to handle=] [=set/contains=] <code>fetch</code>, and |registration|'s [=active worker=]'s [=all fetch listeners are empty flag=] is not set then: