fix(admin): harden stream_route superior_id dependency checks

[shreemaan-abhishek]

Description

Follow-up to #12794, which added dependency-protocol checking and delete protection for stream routes. This PR hardens those checks against a few edge cases:

• Reject self-reference: a route that sets its own id as protocol.superior_id would otherwise pass the protocol-match check on update (it matches itself). Now rejected at write time.
• Fail closed on JSON decode errors: core.json.decode returns nil, err on malformed data. Previously a decode failure silently skipped the record, which could hide a live superior_id reference (allowing deletion of an in-use route) or bypass the protocol-mismatch check. Now it returns an error instead of continuing.
• Explicit status on etcd failure: delete_checker returned nil as the status code on an etcd fetch failure, which surfaces as an unexpected error path. Now returns 503.

Tests

Added two cases to t/admin/stream-routes-subordinate.t:

• superior_id references itself → 400
• force-delete of a referenced superior route → succeeds (bypasses the reference check)

Existing cases are unchanged.