refactor(clientonly): modernize code structure and add comprehensive tests

[KristjanESPERANTO]

This PR improves clientonly start option with better code structure, validation, and comprehensive test coverage.

Changes

Refactoring:

• Improved parameter handling with explicit function parameter passing instead of closure
• Added port validation (1-65535) with proper NaN handling
• Removed unnecessary IIFE wrapper (Node.js modules are already scoped)
• Extracted getCommandLineParameter as a reusable top-level function
• Enhanced error reporting with better error messages
• Added connection logging for debugging

Testing:

• Added comprehensive e2e tests for parameter validation
• Test coverage for missing/incomplete parameters
• Test coverage for local address rejection (localhost, 127.0.0.1, ::1, ::ffff:127.0.0.1)
• Test coverage for port validation (invalid ranges, non-numeric values)
• Test coverage for TLS flag parsing
• Integration test with running server

Testing

All tests pass:

npm test -- tests/e2e/clientonly_spec.js
# ✓ 18 tests passed

[khassel]

Not directly related to this pull request, but I've been wondering whether we need these subdirectories for clientonly and serveronly. We could put everything under js and simply change the call in package.json for serveronly. And we could do the same for clientonly.

But maybe I've overlooked something that argues against it...

[KristjanESPERANTO]

I'm a bit divided on this.

For client-only users, this would be a breaking change. Until now, they have been using
node clientonly --address 192.168.1.5 --port 8080
to start. After a migration like suggested, it would be
node js/client.js --address 192.168.1.5 --port 8080 or
node --run client -- --address 192.168.1.5 --port 8080.

For server users it would stay the same command.

[khassel]

yes, I know that it would be a breaking change for client users, so we better don't change it.

[KristjanESPERANTO]

Yes, if we had strong arguments for a breaking change, we could certainly make the change, but I don't see them here yet.