chore: upgrade api7 and gateway to v3.9.15

[jarvis9443]

Upgrade the api7 and gateway charts to API7 Enterprise v3.9.15 (3.9 release line).

Version bumps

• api7: chart 3.9.0 → 3.9.1, appVersion → 3.9.15, image tags → v3.9.15 (integrated, dp_manager, file_server, developer_portal)
• gateway: chart 3.9.1 → 3.9.2, appVersion → 3.9.15, image tag → 3.9.15

Structural change

• gateway: expose per-port PROXY protocol for stream TCP. Each gateway.stream.tcp map entry now accepts proxy_protocol and proxy_protocol_to_upstream, rendered into stream_proxy.tcp; these override the global proxy_protocol.enable_tcp_pp* defaults. The commented global section and the gateway.stream.tcp example were updated accordingly. READMEs regenerated via make helm-docs.

No Control Plane chart structural changes in this release.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit unit tests in branch chore/upgrade-3.9.15

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