Fix: Correct return type and preserve response timing info

[yoshisatoyanagisawa]

This pull request 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.

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[yoshisatoyanagisawa]

@quasi-mod Sorry for overlooking them in your previous pull request. Let me fix.

[quasi-mod]

Oops, thanks for fixing! LGTM.