test: compare benchmark inventory identities (Fixes #551)

Reject same-count environment and manager replacements across measured and diagnostic refreshes. Preserve duplicates, normalize path keys outside request timing, and keep identity paths out of mismatch diagnostics. Metric schemas and budgets remain unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: karthiknadig