fix: cross-browser test determinism and missing FetchTransport coverage

[joaquin-diaz]

What problem is this solving?

Two unrelated test gaps found while working in this area:

1. EmbracePageManager.test.ts's soft-navigation zero-time tests were flaky on Firefox and WebKit:
   • FakeNavigation.fire() constructed a currententrychange Event and invoked listeners directly instead of dispatching it through a real EventTarget. Some engines only populate Event.timeStamp on actual dispatch, leaving it at 0 otherwise.
   • The "advances the SDK zero time on soft navigation" test compared getZeroTime() before/after with no time gap in between, which could land in the same rounded timestamp bucket under WebKit's coarser timer resolution.

[github-actions]

Chrome DevTools Protocol Tracing (Script: 113.03ms, Heap: 13.14MB)

	Number of Requests	Size of Requests	Script Duration	Task Duration	Heap Used Size
Requests	+13 requests	+42.78 KB
Page Loaded			+20.59 ms	+21.69 ms	+1.19 MB
Generate 100 fetch requests			+20.29 ms	+79.34 ms	+1.64 MB
Generate 100 XHR requests			+38.01 ms	+95.27 ms	+2.85 MB
Click 100 buttons and generate 100 logs			+24.99 ms	+48.18 ms	+2.58 MB
Throw a 100 exceptions			-0.25 ms	+38.96 ms	+2.66 MB
End Session			+9.40 ms	+35.15 ms	+2.22 MB
Total	+13 requests	+42.78 KB	+113.03 ms	+318.60 ms	+13.14 MB

Lighthouse (Script Eval: 128.13ms)

	Difference	Description
Total Blocking Time	+3 ms	Difference in Total Blocking Time: Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds. Learn more about the Total Blocking Time metric.
Main Thread Time	+101.44 ms	Difference in Main Thread Time: Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to minimize main-thread work
Script Evaluation Time	+128.13 ms	Difference in Script Evaluation Time: Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to reduce Javascript execution time.

Platform Tests (vite-7 es2015 gzip: 70.77KB)

vite-6 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-6 - es2015	+209.27 KB	+70.83 KB

vite-7 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-7 - es2015	+209.27 KB	+70.77 KB

webpack-5 Platform Tests

	Total Uncompressed Size	Total Gzip Size
webpack-5 - es2015	0 KB	0 KB

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (49ebdb2) to head (316a3ed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1430      +/-   ##
==========================================
+ Coverage   96.74%   96.87%   +0.12%     
==========================================
  Files         233      233              
  Lines       10183    10183              
  Branches     1437     1440       +3     
==========================================
+ Hits         9852     9865      +13     
+ Misses        329      316      -13     
  Partials        2        2              

see 1 file with indirect coverage changes

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