MCPJVM-372: Public transitional backfill action support for correlation_state

[nimbly-dev]

Description

While transitional legacy import support exists internally, Regression Suite may need a public MCP maintenance action so users and orchestrators can trigger controlled compatibility import without introducing a new top-level tool. This story exposes transitional backfill through artifact_management for correlation_state migration support.

This story should define the public request/response shape, source selection behavior, summary output, and fail-closed behavior for transitional backfill.

Scope

In scope:

• Expose public transitional backfill action support through artifact_management for correlation_state
• Support bounded import from legacy correlation-index.json
• Return deterministic summary output rather than unbounded raw import dumps
• Preserve fail-closed behavior when required deterministic legacy data is missing or invalid
• Keep the public contract explicit that this is transitional migration support only

Out of scope:

• Broad all-state migration semantics
• Storage implementation internals of backfill logic
• Long-term endorsement of legacy shared index data as a source of truth
• Manual mutation of runtime-owned rows outside controlled maintenance actions

Acceptance Criteria

• Public transitional backfill action is available through artifact_management for correlation_state
• Backfill request/response shape is bounded, deterministic, and fail-closed
• Response output provides a usable migration summary for agents/users
• The public contract clearly positions backfill as temporary compatibility support rather than long-term recovery
• The public action does not require a new top-level MCP tool

Implementation Notes

• Keep backfill explicitly narrower than rebuild; do not imply full state-surface reconstruction from legacy correlation index data.
• Public response should make the transitional nature visible.

Testing

• UT for backfill input validation and legacy-source normalization
• UT for summary response shaping and transitional-status wording
• IT for public legacy correlation index backfill invocation
• IT for fail-closed behavior on malformed or incomplete legacy index data