[wangyinz]

Fixes #815.

Depends on #955. This PR is based on the #955 branch so reviewers see only the Issue #815 changes. Retarget it to master after #955 merges.

Scope

• Complete schema metadata preflight before creating sample data.
• Persist the SAVED history node from a history-only copy.
• Keep the caller history unchanged until the waveform document and both history references are durable.
• On failure, remove only invocation-created resources whose identifiers are known: waveform, history, elog, and GridFS documents.
• Surface cleanup failures instead of hiding them.

Intentionally out of scope

• Distributed overwrite, which Make GridFS waveform replacement failure-safe #955 rejects explicitly.
• Cross-resource transactions or ensemble-wide all-or-nothing commits.
• Truncating shared data files. A failed final database write can leave unreferenced appended bytes; removing them safely requires a separate file-layout design.

Verification

• Black, Python compilation, and whitespace checks passed.
• All 32 focused contract tests were collected.
• 3 connection-free contract tests passed locally.
• 29 MongoDB-backed cases were skipped because no local MongoDB service is available; CI must execute them before this draft becomes ready.

[wangyinz]

Review status: no maintainer behavior decision remains. This PR is Draft only because the 29 MongoDB-backed contract cases could not run locally and, while stacked on #955, GitHub currently runs only the Black check. After #955 merges, retarget this PR to master, run the full required CI set, and mark it Ready if those checks pass.

[wangyinz]

Superseded by #1002. GitHub automatically closed this stacked PR when #955 merged and deleted its base branch, and GitHub does not allow it to be reopened after the required head rebase. #1002 contains the same reviewed narrow change rebased onto master.