test: Add default arm to writes_in_flight case as defensive guard

[DanielKellerM]

Summary

Adds a defensive default: $fatal(...) arm to the case(now.dst_protocol) block in tb_idma_backend.sv.tpl. Without it, an out-of-set dst_protocol would leave the id variable undefined and the subsequent writes_in_flight[now.dst_protocol][id] lookup would index with X bits.

Context

The underlying bug that surfaced the need for this guard — random job-file generator (in nonfree) emitting src_protocol=0/dst_protocol=0 unconditionally, which trips the existing $fatal for variants where AXI isn't a valid source/destination — is fixed at the source in a companion nonfree commit (gen_random_jobs: Emit per-variant protocol IDs, not hardcoded 0). After that, the existing $fatal checks at the request and ack loops are the canonical correctness gate. This PR adds the case-default arm as belt-and-suspenders for any future codepath that bypasses those checks.

Test plan

• make idma_hw_all regenerates target/rtl/tb_idma_generated.sv with the new default: arm at each case(now.dst_protocol) instance
• Internal CI run: existing variants continue to pass; coverage of the new default: arm via deliberately-invalid stimulus (separate testbench task)

[Copilot]

Pull request overview

This PR updates the tb_idma_backend testbench template so the response/ack loop handles out-of-set src_protocol / dst_protocol values the same way as the existing request-dispatch and watchdog paths, preventing unnecessary $fatal terminations in variants where the default protocol enum value is not part of the supported protocol set.

Changes:

• Replaced $fatal checks in the ack/response loop with silent coercion to the last valid used_read_protocols / used_write_protocols enum (matching the request and watchdog loops).
• Ensures randomized job runs don’t fail solely due to default-initialized protocol enums not being legal for a given variant.

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