DevTools: Use ScopedObservation for ServiceWorkerDevToolsManager observers

Modernize the observer pattern for `ServiceWorkerDevToolsManager` by
replacing manual `AddObserver`/`RemoveObserver` calls with the safer
`base::ScopedObservation`.

This change updates all implementers (`BrowserAutoAttacher`,
`ServiceWorkerAutoAttacher`) to use the new pattern for improved
lifetime management and code consistency. The base `Observer` has also
been updated to inherit from `CheckedObserver` and enforce a stricter
interface contract with pure virtual methods.

Bug: 452536817
Change-Id: I840a3dd3604bc52323cdf93a9fbcad74b7c623a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7048003
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Wang Neden <nedenwang@gmail.com>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1531281}