Suggested test: copilot/mcp-large-json-payload

## Motivation

Link to gh-aw PR: github/gh-aw#57253 — "Prevent large MCP query payloads from exceeding argument limits" fixed a real production failure where paginated GitHub query results (90-day windows) exceeded OS argument-size limits during `jq --argjson` merging, causing a daily workflow to fail. The fix stores pages in temp files and merges via file operands instead of command args. This class of bug (large-payload/argument-size handling in MCP tool calls) has no regression coverage in this harness.

## Proposed test

- **Workflow file**: `test-copilot-mcp-large-payload.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot
- **Safe output**: `create-issue`
- **Variant**: standard

## Minimal test prompt sketch

Use an MCP tool (e.g. the existing custom `mcp-printEnv`/`mcp-code-quality` fixture, or a small custom MCP server) that returns/receives a large JSON payload (several hundred KB, generated inline), forcing the harness to move data through the safe-outputs/tool-call path without hitting argument-size limits. Report success via `create-issue` once the round-trip completes.

## New fixtures or secrets needed

Possibly a small custom MCP server fixture that echoes back a large synthetic payload (can reuse `.github/workflows/shared/mcp/` patterns). None needed from GitHub/secrets side.

## Notes

This is somewhat implementation-specific to `gh-aw`'s internal query scripts (not the compiled workflow itself), so value is more about the general capability of the harness (large jq/MCP payloads) than reproducing the exact upstream bug. Flagging as an issue rather than a PR since the right minimal fixture/tool to trigger a comparable payload size in this repo is not yet clear.

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/33488119915) · copilot · auto · 29.7 AIC · ⌖ 4.64 AIC · ⊞ 8.8K · [◷](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 with the proposed test-copilot-mcp-large-payload.md and the existing mcp-printEnv/mcp-code-quality fixtures, then review .github/workflows/shared/mcp/ patterns. Run the workflow with workflow_dispatch and verify that a several-hundred-KB JSON payload completes the MCP and safe-outputs path without argument-size failure. Done means the workflow reports success through create-issue.

Written by the indexing model from the issue text.