Fix MCP Gateway mount propagation in large-payload-tester workflow

[Copilot]

The large-payload-tester workflow creates test data in /tmp/mcp-test-fs and expects the filesystem MCP server to read it, but the gateway container had no access to mount it through.

Changes

• Added /tmp/mcp-test-fs:/tmp/mcp-test-fs:ro to sandbox.mcp.mounts in workflow frontmatter
• Recompiled lock file with updated docker command

Flow

# Host creates: /tmp/mcp-test-fs/large-test-file.json
# 
# Gateway container needs:
sandbox:
  mcp:
    mounts:
      - "/tmp/mcp-test-fs:/tmp/mcp-test-fs:ro"  # ← Added
      
# So it can launch filesystem MCP with:
mcp-servers:
  filesystem:
    mounts:
      - "/tmp/mcp-test-fs:/workspace/test-data:ro"  # ← Now works

Gateway launches backend MCP servers via Docker-in-Docker. Without host directory access, it can't propagate mounts to child containers.

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[Copilot]

Pull request overview

This PR fixes a mount propagation issue in the large-payload-tester workflow where the MCP Gateway container couldn't access the /tmp/mcp-test-fs directory to mount it into the filesystem MCP server. The gateway launches backend MCP servers via Docker-in-Docker, requiring host directory access to propagate mounts to child containers.

Changes:

• Added /tmp/mcp-test-fs:/tmp/mcp-test-fs:ro mount to sandbox.mcp.mounts configuration
• Recompiled lock file with updated Docker command and frontmatter hash

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/large-payload-tester.md	Added read-only mount for /tmp/mcp-test-fs to gateway container sandbox configuration
.github/workflows/large-payload-tester.lock.yml	Updated compiled Docker command to include new mount and refreshed frontmatter hash

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