Fix CMUX role acceptance receipt fixture

[teamleaderleo]

Repairs the current-main Verify regression introduced by the merge interaction between #1084 and #1088.

Production validate_acceptance_receipt() correctly requires the expanded receipt identity. The execution-role test fixture still emitted the older shape, so current main push Verify 35646563844 and dependent PRs fail in workspace bootstrap before Rust checks.

This one-file test-only repair adds the six now-required fields using closed fixture values and the current fleet contract generation. No production CMUX role, fleet, execution, routing, lease, or admission semantics change.

Expected focused discriminator: python3 scripts/test-cmux-execution-roles.py.

Related: #1084, #1088.

[teamleaderleo]

Low-risk exact-head self-review complete on 4896ba86de374c958b5351bce8c12d86e580d4fe: the full diff is one test fixture, adding only the six acceptance-receipt fields already required by production validate_acceptance_receipt(). No production CMUX role/fleet/execution/admission/lease code changes. Verify 35647284483 and Cultist 35647284473 are green; zero review threads; PR is mergeable.