fix(extensions): preserve missing error status - #7516

Changes

• preserve NotFound, NotInstalled, and CredentialNotFound as distinct shared integration errors
• map all three missing-resource variants to HTTP 404 without changing their operator-facing text
• add shared-type, bridge, and API-status regression coverage
• add the required changelog fragment

Audit finding: F-5ed431ba4f828858.

Verification

• mise exec -- cargo test -p librefang-types --lib integration::tests -- --nocapture
• mise exec -- cargo test -p librefang-extensions --lib extension_error_maps_to_integration_error -- --nocapture
• mise exec -- cargo test -p librefang-api --lib missing_extension_resources_return_not_found -- --nocapture
• mise exec -- cargo clippy -p librefang-types -p librefang-extensions -p librefang-api --lib -- -D warnings
• mise exec -- cargo fmt --check
• git diff --check
• python3 scripts/check-changelog-attribution.py --base origin/main --head HEAD

Out of scope

• vault, installation-conflict, and generic failure mappings are unchanged
• API response-body scrubbing for internal errors is unchanged