Bump firewall to v0.29.7 and MCP gateway to v0.5.4 #75108

Summary

Update gh-aw's runtime dependency versions to:

• Firewall (gh-aw-firewall): v0.29.7 — https://github.com/github/gh-aw-firewall/releases/tag/v0.29.7
• MCP gateway (gh-aw-mcpg): v0.5.4 — https://github.com/github/gh-aw-mcpg/releases/tag/v0.5.4

Acceptance criteria

• Update the firewall version pin to v0.29.7 and the MCP gateway version pin to v0.5.4.
• Update associated image references or digests, fixtures, and documentation as applicable.
• Regenerate affected compiled workflows through the supported compilation/update path.
• Run the relevant validation and tests to confirm compatibility.