Fix ACA sandbox history reads

[Laveesh Rohra (larohra)]

Purpose

• Fix ACA Sandbox GET /agents/{slug}/history silently returning an empty transcript for sessions with real history.
• Read the existing owner-bound, pointer-selected sandbox checkpoint without creating another history file or external transcript copy.
• Return explicit 404 session_not_found, 410 history_gone, and 503 history_unavailable outcomes while preserving the default Blob-backed in-language path.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

python -m ruff check src tests
python -m mypy src
python -m pytest tests -q

Local gate: 1813 passed, 60 skipped, 82 deselected. Default ACA live smoke skips cleanly when not enabled.

What to Check

• ACA history reuses session/current and checkpoint conversation.json; it creates no second transcript file or Blob/Table projection.
• Owner partition and live sandbox binding are verified before file access.
• Suspended backing resumes on read without touching idle retention.
• First and second turns preserve canonical user/assistant ordering and latest-200 truncation.
• Missing or corrupt retained history is explicit rather than a silent empty success.
• The non-ACA Blob history path and four-method backend seam remain unchanged.

Other Information

• Targets feature/aca-sandboxes; intentionally separate from PR [FRD 0008 U3] Complete ACA sandbox GA qualification #160.
• FRD 0008 Decisions 152-163 record the approved architecture.
• Deployed ACA proof is still pending on Linux/WSL x86_64 with a matching CPython ABI. Verifying the absence of external transcript copies additionally requires scoped authenticated storage-audit access.