fix: preserve final response in PassthroughErrorHandler

[happysnaker]

Description

• make PassthroughErrorHandler return the terminal *http.Response without a competing error when a response exists
• keep http.Client / StandardClient() callers from silently discarding the final HTTP response on retry exhaustion
• add focused tests for the error handler itself and the StandardClient / RoundTripper path

Related Issue

Closes #156

How Has This Been Tested?

• added unit coverage for PassthroughErrorHandler
• added an integration-style test covering StandardClient() preserving the final 500 response

[hashicorp-cla-app]

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[hashicorp-cla-app]

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[happysnaker]

This should now be ready for review on my side. The current status check blocker is the HashiCorp CLA gate; I have already signed the CLA before, but the new PR still shows license/cla as pending right after creation, so I am waiting for that bot/status to refresh.

Change summary:

• PassthroughErrorHandler now returns the terminal *http.Response without a competing error when a response exists
• added a focused unit test for the handler
• added a StandardClient() / RoundTripper test showing the final 500 response body remains available to callers

If you want, I can also add a short README/doc note clarifying this default behavior for PassthroughErrorHandler.

[happysnaker]

Follow-up: I force-pushed the branch so the commit is now authored with my GitHub-linked noreply address (73147033+happysnaker@users.noreply.github.com) instead of the previous unlinked work email.

If the CLA check does not refresh automatically from that updated commit metadata, please let me know and I can re-run whatever recheck flow you prefer.

[happysnaker]

Quick follow-up on the remaining blocker: the commits on these branches now use my GitHub-linked noreply address, and I've already completed the HashiCorp CLA flow, but license/cla is still stuck in PENDING on #288 / #289 / #290.

If there is a maintainer-side recheck I should trigger, or a preferred support path for a stuck CLA status, I'd appreciate a pointer. Thanks!

[happysnaker]

Update: I re-ran the HashiCorp CLA flow successfully and the CLA backend now reports my signature as active for this repo/PR context. If the GitHub license/cla status remains stuck in PENDING, it looks like a sync/recheck issue rather than an unsigned CLA on my side.