test: own sdkharness conformance and resilience

[sophiecarreras]

Summary\n\n- move all 33 b2sdk conformance assertions and all 16 resilience assertions from demand-side-ai/sdkharness into this repository\n- expose them through the existing versioned .sdkharness/tests.tsv contract\n- add simulator-only dispatchers that pin imports to this exact checkout, reject unsafe endpoints and malformed results, and scrub ambient B2 credentials\n- preserve central ownership of scenario inventory, simulator lifecycle, orchestration, evidence, issue reconciliation, reporting, and notification\n- add focused contract tests and an infrastructure changelog fragment\n\n## Verified behavior against fresh local simulators\n\n- conformance: 32 PASS / 1 existing SDK FAIL\n - large.resume: the rerun finishes a different file ID than the unfinished upload\n- resilience: 13 PASS / 2 existing SDK FAIL / 1 expected SKIP\n - api.retry_after_503: retries after about 1 second instead of the requested 2 seconds\n - upload.retry_408: UnknownError reaches the caller after one attempt\n - upload.stall: no documented client timeout option\n\nThe outcomes match the pre-migration standing harness behavior; no contract or setup failures remain.\n\n## Verification\n\n- focused sdkharness unit tests: 17 passed\n- exact repository lint/license gate: uv run --group nox nox -vs lint passed\n- explicit ruff check over every hidden .sdkharness executable: passed\n- direct py_compile over every migrated executable: passed\n- final fresh-simulator run of all 49 scenarios: completed with the counts above\n- manifest: 50 rows, 0 malformed rows, 0 duplicate level/scenario pairs\n- git diff --check: passed\n\nCloses #612\nParent roadmap: backblaze-labs/demand-side-ai#744

[sophiecarreras]

CI classification: the new sdkharness contract tests and the repository unit suites pass on the completed macOS/Python 3.14 job. The job turns red only in the later credentialed real-B2 integration stage, where 12 existing encryption tests expect SSE-B2 but the configured account returns NONE. This PR does not touch SDK behavior or test/integration.\n\nTracked independently in #614. I am treating that as an observed upstream/live-suite finding, not as a migration regression.