net: Parameterize ReconsiderProxyAfterError tests

The ReconsiderProxyAfterError tests previously iterated through a list
of network errors within a single test. This CL refactors them into a
parameterized test suite to improve failure diagnostics and isolation.

The bulk of the change introduces the necessary structs and
instantiation for the parameterized tests. I also took the opportunity
to make the histogram metric verification more precise by using
`EXPECT_THAT(..., BucketsAre(...))`. The remaining diff consists of
minor whitespace changes from removing the old for loop.

This is a test-only refactoring with no change to production code.

Bug: 422740116
Change-Id: I3e5d5996d38525edca3ba3fa5b8347cb524f331c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781562
Commit-Queue: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Reviewed-by: Andrew Williams <awillia@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1492006}