Suggested test: copilot/sandbox-mounts-from-partial

## Motivation

Link to the gh-aw PR: github/gh-aw#44500 — Allow shared workflow partials to define and merge `sandbox.agent.mounts`

This PR enables shared workflow partials to carry `sandbox.agent.mounts` entries, which are merged (with deduplication) into the consuming main workflow at compile time. Previously, partials could not declare sandbox config. No existing test exercises this merge behaviour.

## Proposed test

- **Workflow file**: `test-copilot-sandbox-mounts-from-partial.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot
- **Safe output**: `create-issue` (simple output to confirm the workflow ran successfully)
- **Variant**: standard

## Minimal test prompt sketch

Create a shared partial `.github/workflows/shared/mounts-partial.md` that declares `sandbox.agent.mounts` with one bind mount. The main workflow imports it and declares its own overlapping mount. The agent creates a GitHub issue confirming it ran. The test asserts the compiled lock file merges both mount declarations without duplication.

## New fixtures or secrets needed

A new shared partial file in `.github/workflows/shared/` of the `githubnext/gh-aw-test` repo is needed. No new secrets required.

## Notes

This is a compiler/merge behaviour test — the assertion is on the compiled `.lock.yml` (mounts deduplicated) plus runtime success. Overlaps conceptually with `test-copilot-inline-sub-agents.md` but exercises a different frontmatter surface.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/29083912426) · 20.8 AIC · ⌖ 5.52 AIC · ⊞ 5.6K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-test+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fgh-aw-test%2Fsuggest-new-e2e-tests%22&type=issues)

Start by reading the existing test-copilot-inline-sub-agents.md and the workflow test conventions. Add the named test file and the shared .github/workflows/shared/mounts-partial.md fixture, then run the relevant end-to-end test. Done means the compiled .lock.yml contains both mount declarations without duplication and the workflow completes with a create-issue output.

Written by the indexing model from the issue text.