fix: snapshot published runtime artifacts

[1aal]

Summary

• publish report artifacts by stable relative file path instead of content digest so an appending/re-written report appears as one asset
• copy the latest discovered content into .published/<artifactId>/ before advertising metadata to Cloud
• serve artifact IDs from .published snapshots before falling back to the legacy live-directory scan, and skip .published during discovery

Behavior

• A file modified multiple times in one run keeps one artifact ID/card and serves the latest published content.
• Different report files still become different artifacts.
• The legacy live scan remains as fallback for existing links and old Runtime data.

Verification

• uv run pytest tests/runtime_manager/test_report_artifacts.py
• uv run pytest tests/runtime_manager
• uv run ruff check runtime_manager/artifacts.py tests/runtime_manager/test_report_artifacts.py
• python3 -m py_compile runtime_manager/artifacts.py
• git diff --check
• git diff --check origin/apecloud-base...HEAD