Limit exposed fields from getnetworkinfo to prevent metadata disclosure

[NateIsern]

Motivation

• The public /api/network-info endpoint previously returned the raw getnetworkinfo RPC response, which exposes node connectivity and deployment metadata unnecessary for the UI.
• The frontend only consumes a few fields (connections/subversion/version/relayfee), so responses should be allowlisted to prevent fingerprinting and information disclosure.

Description

• Add server/lib/network-info.ts with toPublicNetworkInfo and PublicNetworkInfo that validate and return only version, subversion, protocolversion, connections, and relayfee.
• Update the server /api/network-info handler in server/index.ts to call toPublicNetworkInfo(blockCache.getNetworkInfo(network)) before responding.
• Add server/lib/network-info.test.ts regression tests asserting that sensitive fields such as networks, localaddresses, and warnings are not exposed and malformed values are ignored.
• Preserve existing API behavior for consumers while reducing the surface area of exposed node metadata.

Testing

• Ran npm test and all tests passed (5 files, 44 tests passed).
• Ran npm run typecheck and the TypeScript build/type-check completed successfully.
• An initial npm test -- --runInBand invocation failed because vitest does not support the --runInBand flag, but this flag is unnecessary and the standard test run succeeded.

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Codex Task