sdkharness: fail, not skip, on auth/connection/import errors; scrub proxies

[sophiecarreras]

Summary

Review findings for the repo-owned .sdkharness/ Python checks.

• FAIL, not SKIP. The 33 conformance leaves mapped Unauthorized/InvalidAuthToken/AccessDenied and gaierror/ConnectionError/ServiceError/B2ConnectionError to COULD-NOT-RUN, and any ImportError to missing-runtime. lib/contract.py turned that into SKIP, exit 0. At a loopback simulator the harness supplies the server and the credential, so these are all defects now. Only ModuleNotFoundError for b2sdk itself (and a missing simulator URL) may be COULD-NOT-RUN; a broken or circular SDK import is FAIL. The urls.native_download URLError paths and the client.auth_persistence second-process mapping follow the same rule.
• Dispatcher SKIP policy. lib/contract.py only accepts no-realm-option, missing-runtime, no-client-option, not-claimed as SKIP; any other COULD-NOT-RUN reason becomes FAIL (the CLI's "SKIP policy FAIL"). The resilience leaves get the same import handling.
• Proxy scrub. HTTP_PROXY/HTTPS_PROXY/ALL_PROXY/FTP_PROXY (any case) are removed and NO_PROXY is pinned to 127.0.0.1 in the dispatcher child environment, in the health check, and in every leaf (via loopback_guard.refusal()), so a proxy on the machine cannot turn a passing loopback scenario into an "unreachable" one.
• Docs. README no longer says api.retry_after_503 / upload.retry_408 FAIL (both pass); the garbled files.upload docstring is fixed; references to harness-only paths (bin/conformance/README.md, docs/sdks/.../card.md, bin/resilience/...) are removed.
• Changelog fragment changelog.d/+sdkharness-fail-not-skip.infrastructure.md.

Test plan

• New test/unit/test_sdkharness_fail_not_skip.py: a dead loopback port and an SDK ImportError each yield FAIL (leaf and dispatcher translation); a genuinely missing b2sdk is still SKIP; unreachable/unauthorized reasons from the leaf are FAIL in the dispatcher; proxy variables are scrubbed.
• pytest test/unit/test_sdkharness*.py: 116 passed.
• Ran all 33 conformance + 16 resilience scenarios and the health check against a local loopback b2-simulator with hostile HTTP_PROXY/HTTPS_PROXY/ALL_PROXY set: no production endpoint was contacted.

Follow-up outside this repo: the central Python conformance adapter (python-repository-conformance-adapter.sh in the harness) still leaves SKIP_POLICY at the amber default and should set SKIP_POLICY=fail.

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