Complete resilient Data Management migrations

[paullizer]

Summary

• Completes issue Make Data Management migrations resilient for large blob transfers with cancel, resume, and incremental sync #1043 on the current Development baseline with durable Data Management migration checkpoints, cancel/retry recovery, incremental modes, server-owned preview/reconciliation, and resumable Azure AI Search keyset paging.
• Adds target-side coordination and Search writer fencing across independent SimpleChat sources, including response-loss quarantine, guarded mirror deletes, and explicit external-writer freeze acknowledgement.
• Keeps personal unshare authorization fail-closed: Search ACL projection must be acknowledged before Cosmos access is revoked; a fenced projection returns retryable 503.
• Adds migration provenance, manifests, capacity recovery, admin workflow controls, functional coverage, scripts, and Development-aligned v0.250.067 documentation/release notes.

Validation

• 85 passed, 1 deselected: focused Make Data Management migrations resilient for large blob transfers with cancel, resume, and incremental sync #1043 migration suite on this Development-based branch. The deselection is the intentionally excluded environment browser case.
• 16 passed: route-policy and migration-script contracts.
• Changed Python compilation, JavaScript syntax, whitespace checks, and VS Code diagnostics passed.
• Branch ancestry verified: origin/Development is the direct ancestor; this PR contains only commit 8ea6238b from the local Make Data Management migrations resilient for large blob transfers with cancel, resume, and incremental sync #1043 implementation.

Closes #1043