refactor(ipc): share the IPC classification gate and board test mock

[pauliuszaleckas]

Groundwork for a board graphic-delete tool, useful on its own.

• with_ipc_classified moves to tools/mod.rs. It was about to exist twice
  verbatim, and it is the gate deciding whether a board file may be edited
  behind a live KiCad — the one helper whose copies must not drift. Related
  context in Closed-board fallback cannot tell "KiCAD was never running" from "KiCAD just died holding this board" #240.
• get_items_of_types_in asks for several object types in one GetItems. The
  field is repeated and every send_command dials a fresh socket, so four types
  cost one round trip instead of four. A non-OK status is now an error rather
  than an empty result — previously a failed request was indistinguishable from
  an empty board, and one failure zeroed every type in the batch.
• delete_items_in addresses a named document, like the other _in methods.
  delete_items delegates to it; deleting from the first open board rather than
  the requested one was the same hazard the reader fixes hit.
• pcb_board's tests get one mock KiCad (board_mock) owning the socket, the
  envelope, and the GetOpenDocuments answer, so a test writes only its own
  command arms. Three duplicate ToolContext builders go with it.

No tool surface, argument, or response shape changes.

Gate run locally on this commit alone: cargo test --workspace --locked --lib --tests, cargo test --workspace --locked --doc, cargo clippy --workspace --locked -- -D warnings, cargo fmt --all -- --check.