Bump MCP Gateway to v0.4.14

[lpcox]

Summary

Update gh-aw's default MCP Gateway dependency from v0.4.13 to v0.4.14.

Required changes

• Update constants.DefaultMCPGatewayVersion in pkg/constants/version_constants.go to v0.4.14.
• Update the corresponding expectation in pkg/constants/version_constants_test.go.
• Refresh container image digest pins for ghcr.io/github/gh-aw-mcpg:v0.4.14.
• Recompile and commit affected generated workflow lock files and golden fixtures.
• Update any version snapshots, manifests, or documentation that explicitly reference the previous default v0.4.13.

Acceptance criteria

• Newly compiled workflows use ghcr.io/github/gh-aw-mcpg:v0.4.14 by default.
• All generated image references use the verified digest for the v0.4.14 release.
• Version-constant, workflow compilation, schema, and golden-file tests pass.
• No maintained default-version references remain on v0.4.13.