Refactor ServiceWorkerRegistry::FindRegistrationForClientUrl to remove code duplication

Previously, the method had two separate, almost identical, code paths
controlled by the kServiceWorkerMergeFindRegistrationForClientUrl
feature flag. This resulted in significant code duplication, making the
code harder to read and maintain.

This change unifies these two branches into a single, shared
implementation. The logic specific to the feature flag is handled at the
beginning of the function, while the core logic for finding a
registration is now shared.

This refactoring improves code readability and maintainability with no
change in behavior.

Bug: 429060039
Change-Id: I7fb94586418950be1b35afb8d064fb18095abf96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6698572
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1481994}