fix(session): allow clearing archived timestamp via HTTP API

[kjhq]

Issue for this PR

Closes #12393
Unblocks #16030

Type of change

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

What does this PR do?

The internal session schema (UpdatedTime at session.ts:302) supports null for the archived field, but the HTTP API UpdatePayload schema rejects it. You can archive a session via the API but cannot unarchive it.

This aligns the HTTP API schema with the internal domain model by accepting null for archived in:

• UpdatePayload (httpapi/groups/session.ts:51)
• SetArchivedInput (session/session.ts:261)
• setArchived function signature and implementation (session/session.ts:729)

The setArchived implementation also coalesces undefined to null because the projector rejects undefined (it requires explicit null to clear a field).

How did you verify your code works?

• Added test: PATCH with time: { archived: null } clears the timestamp and returns archived: undefined
• Added test: unarchived sessions reappear in listGlobal default results
• Ran bun typecheck in packages/opencode — passes
• Ran bun test test/server/httpapi-session.test.ts — 13 pass, 0 fail
• Ran bun test test/server/global-session-list.test.ts — 4 pass, 0 fail

Screenshots / recordings

N/A — not a UI change.

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:

Based on my search, here are the potentially related PRs:

Potentially Related PRs:

1. feat(app): view archived sessions & unarchive #15250 - feat(app): view archived sessions & unarchive

   • Related to unarchiving sessions functionality

2. feat: add unarchive/restore for archived sessions #24154 - feat: add unarchive/restore for archived sessions

   • Also addresses unarchiving/restoring archived sessions

However, PR #27510 (the current PR) appears to be a focused fix addressing a specific API schema alignment issue (allowing null for the archived field), whereas the other PRs seem to be broader feature implementations for unarchiving functionality. These may be complementary rather than duplicates, but they're worth reviewing to ensure there's no overlapping work.

[github-actions]

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