Bump MCPG container image to v0.2.15

[lpcox]

The MCP Gateway container image reference used in compiled workflows (e.g. DIFC_PROXY_IMAGE) is currently pinned to ghcr.io/github/gh-aw-mcpg:v0.2.12.

Please bump it to v0.2.15 so that newly compiled workflows pick up the latest gateway fixes and features.

Current: ghcr.io/github/gh-aw-mcpg:v0.2.12
Target: ghcr.io/github/gh-aw-mcpg:v0.2.15

[github-actions]

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