server: replace only items with identical attributes

[julian-corbet]

CreateItem(replace=true) currently reuses the subset matcher needed by SearchItems. As a result, creating an item with a coarser attribute set removes every more-specific item that contains those attributes.

This change:

• adds exact matching for both encrypted and unencrypted file items
• uses it for single and bulk file-backend replacement
• uses it for the D-Bus server's in-memory item replacement
• keeps subset matching unchanged for searches and explicit query deletion
• adds regressions for the file, bulk, and D-Bus paths, including exact replacement after a coarse item is created

Fixes #541.

[julian-corbet]

Validation update:

• The full fork CI run passes Check, Rustfmt, Clippy, Test Suite (all feature/backend combinations), and Meson: https://github.com/julian-corbet/oo7/actions/runs/31764506620
• cargo-deny is the only failing job. An untouched main baseline has the identical failure: https://github.com/julian-corbet/oo7/actions/runs/31764541659
• Both report the existing wildcard dependency, duplicate syn versions, and RUSTSEC-2026-0244; this patch does not touch dependencies.

The upstream PR workflows are still awaiting first-time-contributor approval.

[bilelmoussaoui]

Both CI failures are unrelated to this one, so ignore them

[julian-corbet]

Review revision validation: fork CI passes Check, Rustfmt, Clippy, the full Test Suite, and Meson. cargo-deny reports the same wildcard dependency, duplicate syn, and RUSTSEC-2026-0244 errors as the untouched main baseline; this revision introduces no new policy errors.