fix(backend): allow admins to download submitted agents pending review (#12535)

## Why

Admins cannot download submitted-but-not-yet-approved agents from
`/admin/marketplace`. Clicking "Download" fails silently with a Server
Components render error. This blocks admins from reviewing agents that
companies have submitted.

## What

Remove the redundant ownership/marketplace check from
`get_graph_as_admin()` that was silently tightened in PR #11323 (Nov
2025). Add regression tests for both the admin download path and the
non-admin marketplace access control.

## How

**Root cause:** In PR #11323, Reinier refactored an inline
`StoreListingVersion` query (which had no status filter) into a call to
`is_graph_published_in_marketplace()` (which requires `submissionStatus:
APPROVED`). This was collateral cleanup — his PR focused on sub-agent
execution permissions — but it broke admin download of pending agents.

**Fix:** Remove the ownership/marketplace check from
`get_graph_as_admin()`, keeping only the null guard. This is safe
because `get_graph_as_admin` is only callable through admin-protected
routes (`requires_admin_user` at router level).

**Tests added:**
- `test_admin_can_access_pending_agent_not_owned` — admin can access a
graph they don't own that isn't APPROVED
- `test_admin_download_pending_agent_with_subagents` — admin export
includes sub-graphs
- `test_get_graph_non_owner_approved_marketplace_agent` — protects PR
#11323: non-owners CAN access APPROVED agents
- `test_get_graph_non_owner_pending_marketplace_agent_denied` — protects
PR #11323: non-owners CANNOT access PENDING agents

### Checklist

- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] 4 regression tests pass locally
  - [x] Admin can download pending agents (verified via unit test)
  - [x] Non-admin marketplace access control preserved

## Test plan
- [ ] Verify admin can download a submitted-but-not-approved agent from
`/admin/marketplace`
- [ ] Verify non-admin users still cannot access admin endpoints
- [ ] Verify the download succeeds without console errors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes access-control behavior for admin graph retrieval; risk is
mitigated by route-level admin auth but misuse of `get_graph_as_admin()`
outside admin-protected routes would expose non-approved graphs.
> 
> **Overview**
> Admins can now download/review **submitted-but-not-approved**
marketplace agents: `get_graph_as_admin()` no longer enforces ownership
or *marketplace APPROVED* checks, only returning `None` when the graph
doesn’t exist.
> 
> Adds regression tests covering the admin download/export path
(including sub-graphs) and confirming non-admin behavior is unchanged:
non-owners can fetch **APPROVED** marketplace graphs but cannot access
**pending** ones.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
a6d2d69ae4e5cd47c4d8dddb5566c3052c8a90d0. This will update automatically
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---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>