(Optional) Language server protocol tests without VS Code

[CatarinaGamboa]

Why: when an integration test fails, it isn't clear whether the server or the client broke. Talking to the server over LSP directly separates the two, runs faster, and needs no display. It overlaps with #131, so only do this if integration failures turn out hard to diagnose.

What: start the server JAR, connect with vscode-jsonrpc, send initialize, didOpen and liquidjava/verify, then check the liquidjava/diagnostics notification for the passing and failing fixtures.

References:

• rust-analyzer's "slow tests" drive its language server over an LSP connection without an editor: https://github.com/rust-lang/rust-analyzer/tree/master/crates/rust-analyzer/tests/slow-tests

[rcosta358]

I recommend keeping this optional and deferring it for now. The integration tests added in #144–#146 already exercise initialization, opening a document, Verify, and explicit passing/failing diagnostic notifications through the bundled extension in real VS Code.

A direct JSON-RPC suite would mainly help isolate server failures from client failures and run without an editor or display. That is useful, but adds another process-launch, connection, event-waiting, and cleanup harness to maintain. The failures encountered while implementing the current suites were diagnosable using logs and targeted inspection.

Revisit this if integration failures repeatedly leave us unsure whether the server or client broke, as the issue already proposes.

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