Suggested test: copilot/assign-to-agent-with-config

## Motivation

Link to the gh-aw PR: github/gh-aw#47047 — fix: send agent_assignment in assignAgentToIssue REST payload

Before this fix, `assign-to-agent` silently dropped `model`, `customInstructions`, `baseBranch`, and `pullRequestRepoSlug` fields — the agent was always assigned with a bare payload. The fix wires these into the API request. There is currently no E2E test that exercises any of these optional config fields.

## Proposed test

- **Workflow file**: `test-copilot-assign-to-agent-with-config.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot
- **Safe output**: `assign-to-agent`
- **Variant**: standard

## Minimal test prompt sketch

Dispatch a workflow that instructs the agent to assign itself to the triggering issue with `baseBranch: main` and a short `customInstructions` string in the frontmatter config. Verify the assignment request carries those fields.

## New fixtures or secrets needed

None. The test targets the triggering issue in `githubnext/gh-aw-test`.

## Notes

Existing `test-copilot-assign-to-agent.md` exercises only the bare assignment path. This test specifically validates that optional config fields (`baseBranch`, `customInstructions`, `model`) are forwarded correctly. A `samples:` block can declare a dummy assignment output to keep it deterministic.

> [!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/29907300322) · 24.9 AIC · ⌖ 5.62 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 comparing the proposed test-copilot-assign-to-agent-with-config.md with the existing test-copilot-assign-to-agent.md. Add the workflow_dispatch test using the copilot engine and the specified configuration fields, then run the E2E test and verify the assignment request carries the optional values.

Written by the indexing model from the issue text.