Consolidate measurement artifact contracts

[yoshifuminakamura]

Summary

• centralize Measurement Artifact path validation and stored filename construction
• reuse the shared helper from ingest, result detail, evidence packet, reuse manifest, and file serving paths
• add contract tests for safe artifact paths, stored filenames, public padata matching, and non-archive upload requirements
• clarify that detailed timing observations are optional measurement evidence until a mapping review promotes them to fom_breakdown

Tests

• .venv/bin/python -m pytest result_server/tests
• bash scripts/tests/test_send_results_profile_data.sh
• bash scripts/tests/test_bk_timing_observations.sh
• bash scripts/tests/test_result_common_json_contract.sh