fix(app-server): restrict proxy endpoint to dev ports

[OnlyYu1996]

Summary

• Restrict /api/v1/proxy/:port and nested proxy paths to the same known dev-server port allowlist used by /api/v1/ports
• Reject non-dev ports such as 22 or 9200 before any localhost request is made
• Add regression coverage for allowed and denied proxy ports

Fixes PlatformNetwork/bounty-challenge#53238.

Validation

• cargo fmt --package cortex-app-server --check
• cargo test --package cortex-app-server proxy::tests -- --nocapture
• cargo check --package cortex-app-server --lib
• git diff --check

[greptile-apps]

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