Deflake the initialize dial-error proxy test

[amirejaz]

Summary

TestRoundTripDoesNotReplayInitializeOnDialError fails intermittently in a full-suite run with Received unexpected error on the HTTP call, while passing every time in isolation (6/6 plain, 4/4 with -race).

It used http.DefaultClient, whose shared transport pools keep-alive connections across this package's parallel tests. Reusing one whose proxy has since been closed surfaces as a transport error before the request is even made.

• Use a dedicated client with DisableKeepAlives.
• Stop requiring NoError on the request itself. For a dial failure either outcome is legitimate — the reverse proxy may synthesize a 502, or the connection may fail outright — so that assertion pinned an incidental detail rather than the behaviour under test.

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

Package green with -race, and the test still fails when the production guard it covers is removed:

--- FAIL: TestRoundTripDoesNotReplayInitializeOnDialError
    the proxy must not send its own initialize to the target service for an initialize dial error

So the relaxed HTTP assertion did not weaken what the test actually guards.

Does this introduce a user-facing change?

No. Test-only.

Special notes for reviewers

Mine from #6152, surfaced in a full-suite run rather than CI. Other tests in this file use http.DefaultClient too and are presumably exposed to the same reuse hazard; I have left them alone rather than widen this beyond the test that actually failed.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.70%. Comparing base (d163115) to head (9d1901c).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6264      +/-   ##
==========================================
- Coverage   72.71%   72.70%   -0.01%     
==========================================
  Files         742      742              
  Lines       77299    77299              
==========================================
- Hits        56209    56204       -5     
- Misses      17126    17134       +8     
+ Partials     3964     3961       -3     

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