LSP malformed JSON can terminate the server loop

[Widthdom]

Summary

LspServer.HandleMessage parses the payload before entering the try block that catches JsonException. A malformed JSON-RPC frame can therefore throw out of Run and terminate the LSP server loop instead of returning a JSON-RPC parse error or dropping the bad frame.

Evidence

• src/CodeIndex/Lsp/LspServer.cs:29 calls HandleMessage(payload) without a surrounding catch.
• src/CodeIndex/Lsp/LspServer.cs:39 runs JsonDocument.Parse(payload) before the later try block.
• The catch at src/CodeIndex/Lsp/LspServer.cs:63 includes JsonException, but parse failures from line 39 never reach it.

Expected

Malformed JSON should be handled inside the LSP message loop and should not terminate the server process.

[Widthdom]

I'm starting work on this in branch fix-issue2829-2830.