Fix pre-existing test compilation failures blocking CI

[Copilot]

Six CLI issues (#702, #703, #708, #709, #681, #682) were already addressed in the codebase. The actual CI blocker was four accumulated test compilation errors across unrelated commits.

Fixes

• pipeline.rs — Remove unused NamedSink<T> struct and use std::io from test module; both triggered -D dead-code/-D unused-imports errors at test compile time

• framed.rs — Fix 3 EOF-flush tests still using FormatProcessor::Passthrough as a unit variant after it became a struct variant in fix: increment parse_errors for invalid JSON lines in passthrough JSON mode #755; change to FormatProcessor::passthrough(Arc::clone(&stats))

• tcp_input.rs — Fix 2 max-line-length tests that assumed client_count() would be observed as >0. When the kernel socket buffer is large enough to deliver the full 1 MiB payload in one shot, the entire connection lifecycle (accept → read → disconnect) completes inside a single poll() call:

  // Before: never true when lifecycle completes in one poll()
  if input.client_count() > 0 { was_connected = true; }
  
  // After: wait for writer thread to signal completion
  if input.client_count() == 0 && writer.is_finished() { break; }
  writer.join().expect("writer thread should have connected and completed");

• conflict_schema.rs — Prefix unused num_rows binding with _ to suppress -D unused-variables in test module

[strawgate]

Closing: the test compilation errors this PR fixed have already been resolved on master (via #783 and other recent merges). All workspace tests pass on current master.