fix: remove redundant cookie writes causing race conditions

[daniel-graham-amplitude]

Summary

PROBLEM

This block of code has a race condition in it:

1. it writes deviceId which triggers updateStorage which won't include lastEventId, lastEventTime, etc... because it hasn't been written yet
2. it writes the rest of the values, which also update storage

If you refresh between #1 and #2 then it won't have lastEventId or lastEventTime in the cookies, which will cause them to be reset the next page and cause a new session_start event.

SOLUTION

This fix adds a variable to disable update storage temporarily.

Checklist

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Note

Medium Risk
Changes when BrowserConfig persists identity/session fields by batching initial writes into a single updateStorage() call, which could affect cookie/state consistency if any consumers relied on intermediate writes. Scope is limited to browser-side session storage updates and a small test/mock adjustment.

Overview
Prevents redundant/partial cookie writes during BrowserConfig construction by temporarily disabling updateStorage() while setters run, then re-enabling and persisting once at the end (guarded via a new _disableStorageUpdate flag).

Adds a note in the context plugin about atomizing event-id/time writes, and updates getTopLevelDomain tests to stop stubbing a removed/unused CookieStorage._isEnabled field.

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