Bound LSP Content-Length and header line sizes

[Widthdom]

Evidence

src/CodeIndex/Lsp/LspServer.cs parses Content-Length into an int and then rents exactly that size:

var buffer = ArrayPool<byte>.Shared.Rent(contentLength);

ReadAsciiLine also builds a List<byte> until newline without a header-line length cap.

Impact

A malformed or hostile LSP peer can advertise a very large Content-Length or send an unbounded header line. Even though LSP is normally local stdio, this can still cause excessive allocation or memory pressure in editor-launched processes.

Expected

Add maximum LSP frame size and header-line limits. Reject frames over the cap with a structured error or close the stream. Prefer clear constants and tests for boundary values.

[Widthdom]

Work started on branch fix-issue2825-2826.