chore: bump MCPG (MCP Gateway) version to v0.2.19

[lpcox]

Summary

Bump the default MCP Gateway (gh-aw-mcpg) Docker image version from v0.2.17 to v0.2.19.

Changes Required

1. Update pkg/constants/version_constants.go: Change DefaultMCPGatewayVersion from "v0.2.17" to "v0.2.19"
2. Add container digest pin: Resolve the SHA256 digest for ghcr.io/github/gh-aw-mcpg:v0.2.19 and add it to .github/aw/actions-lock.json
3. Rebuild and recompile: make build && make recompile to regenerate all lock files with the new version and digest pin
4. Run validation: make agent-finish to ensure no regressions

Context

• Current version: v0.2.17 (defined in pkg/constants/version_constants.go:71)
• Constant: DefaultMCPGatewayVersion
• Docker image: ghcr.io/github/gh-aw-mcpg
• Container pin cache: .github/aw/actions-lock.json

Checklist

• Update version constant
• Resolve and add SHA256 digest pin to actions-lock.json
• Rebuild binary (make build)
• Recompile all workflows (make recompile)
• Verify lock files contain digest-pinned image references
• Run make agent-finish

[copilot-swe-agent]

The agent encountered an error and was unable to start working on this issue: The service is temporarily unavailable. Please try again later. (Request id: A99D:286FBA:11781FEA:11CCD5E4:69DD1D5E)