fix(vprogram): check store dependencies of V-PROGRAM messages at processing time

[odesenfans]

Problem

V-PROGRAM messages had no dependency existence check: VProgramMessageHandler did not override check_dependencies, and find_missing_volumes only branches on ProgramContent / InstanceContent. A V-Program referencing store files unknown to the node was processed successfully and then parked invisibly at the scheduler, whose hard lookup was the only guard.

This is the pyaleph phase-2 work item recorded in the #1222 review notes (the forget-protection half already landed there via get_vms_dependent_volumes).

Fix

Add check_dependencies to VProgramMessageHandler. It checks the file pins for the five store references of the content:

• runtime.ref (runtime manifest)
• workload.ref and workload.hash_tree
• each verified volume's ref and hash_tree

V-Program references are direct store item hashes (there is no use_latest / file tag mechanism for measured content), so the pin check is sufficient. Missing refs raise VmVolumeNotFound, giving the same retry-then-reject behavior as programs with missing volumes (VM_VOLUME_NOT_FOUND).

Out of scope: the balance-removal/GC path not consulting get_vms_dependent_volumes for any executable type is a separate, pre-existing gap.

Tests

• New test_process_vprogram_missing_refs: a V-Program with no seeded refs is rejected with VM_VOLUME_NOT_FOUND and the error details list all five missing refs (written first, watched fail against main).
• Existing processing and API tests now seed the referenced file pins (mirroring insert_volume_refs in the program tests) so they still exercise their original paths (processing, cost persistence, credit rejection, forget blocking).
• tests/message_processing/, tests/api/test_vprogram_api.py, tests/db/test_vprograms_db.py, tests/services/test_cost_service_vprogram.py, tests/test_repair.py, tests/jobs/test_check_removing_messages.py: all passing.

[foxpatch-aleph]

The PR correctly adds a check_dependencies override to VProgramMessageHandler that validates all store references (runtime, workload, hash trees, verified volumes) exist as file pins before processing, mirroring the existing pattern in vm.py. The implementation is straightforward, uses the existing find_file_pins accessor and VmVolumeNotFound exception, and is consistent with the codebase conventions. All tests exercising the processing pipeline are updated to seed the required file pins, and a new test validates the rejection path when refs are missing. No correctness, security, or quality issues found.