Fix RPC free sectors edge case

[n8mgr]

Fixes an edge case in RPCFreeSectors where the swap-and-truncate deletion logic could keep a deleted sector root and lose a kept one when indices overlap with the replacement zone at the tail of the slice. The client now sorts indices descending and deduplicates before sending.

[Copilot]

Pull request overview

Fixes an edge case in RPCFreeSectors where swap-and-truncate deletion could produce incorrect sector-root sets when deletion indices overlap with the tail replacement zone.

Changes:

• Normalize RPCFreeSectors indices client-side by sorting descending and deduplicating before sending/verification.
• Update/add tests to compute expected roots using the same normalized swap behavior and add a regression suite covering overlap cases.
• Add a changeset entry documenting the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
rhp/v4/rpc.go	Normalizes free-sector indices (descending sort + dedupe) to avoid incorrect swaps and proof mismatches.
rhp/v4/rpc_test.go	Adds assertSectorsFreed helper and a table-driven regression test for overlap scenarios; updates existing test to use the helper.
.changeset/...rpcfreesectors...md	Adds a patch-level changelog entry for the fix.

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