Validate critical paths (liblldb, debugserver/lldb-server) and improve error messages

[vaporif]

When critical paths like LLDB_DEBUGSERVER_PATH or liblldb point to non-existent files, lldb errors are cryptic and don't indicate the actual problem.

Example error when debugserver is missing:
executable doesn't exist: '(empty)'

Affected paths

• lldb.library (liblldb) - passed via --liblldb flag
• lldb.server (debugserver on macOS, lldb-server on Linux) - passed via LLDB_DEBUGSERVER_PATH env var

Current behavior

• No validation before launching adapter
• Error messages don't indicate which dependency is missing

Suggested improvement

Validate paths exist before spawning adapter. If missing, return helpful error:

liblldb not found at ''. Check your lldb.library setting.

debugserver/lldb-server not found at ''. Check your lldb.server setting or install debugging tools.

References

• codelldb/extension/novsc/adapter.ts

  Lines 24 to 26 in 39d9acc

  	if (options.liblldb) {
  	args.push('--liblldb', options.liblldb);
  	}

• codelldb/extension/novsc/adapter.ts

  Lines 36 to 37 in 39d9acc

  	if (options.lldbServer)
  	env['LLDB_DEBUGSERVER_PATH'] = options.lldbServer;

Similar pattern already exists for workDir validation in adapter.ts.

Related: #999 (debugserver issues on macOS)