fix(session): recover legacy parts for old sessions

[seilk]

Issue for this PR

Closes #16878

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes one specific old-session failure mode: the session is discoverable and can be opened, but the body renders as metadata-only rows because the SQLite part rows are missing.

In the case I traced locally, widening the session-history window made the old sessions show up again, but opening them only showed assistant headers like Sisyphus · claude-opus-4-5-20251101-max · 29.8s with no actual message body.

The underlying issue is that current reads rely on SQLite part rows, while some older sessions still have recoverable part JSON in legacy on-disk storage. This change adds a narrow read-time fallback in MessageV2 so that when SQL parts are missing, opencode can still hydrate the message from legacy part storage.

How is this different from other recent session fixes?

This PR is intentionally separate from the recent session-list fixes:

• feat(tui): make session history window configurable #17046 fixes session discovery in the TUI when older sessions are filtered out by the 30-day history window
• fix(opencode): lost sessions across worktrees and orphan branches #16389, fix(core): make worktrees read the project id from local workspace #16795, and related issues fix project/worktree/project-id problems that orphan sessions or hide them from the list

This PR does not change how sessions are discovered. It fixes the next failure mode after discovery succeeds: old sessions open, but their bodies are empty because the read path cannot recover missing part rows.

How did you verify your code works?

• reproduced the issue against a real old session where SQLite had message rows but no part rows
• confirmed legacy part files still existed on disk under storage/part/...
• verified the fallback restores parts for that historical message, including text content
• ran typecheck successfully

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The only PR that appears in the search results related to this issue is the current PR (#17054) itself. The other PRs found (16447, 16628, 17046) are tangentially related but address different concerns:

• feat(tui): make session history window configurable #17046 is about making the session history window configurable (related to the context mentioned but a separate feature)
• feat: add startup cleanup and maintenance system #16628 is about cleanup and maintenance
• feat: add conversation-only revert capability #16447 is about conversation revert capability

No duplicate PRs found

[github-actions]

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