Report a non-UTF-8 Lean runtime reply as a protocol error and simplify request phases

[Deicyde]

One fix and three cleanups in servers/lean_client.py (+27/-39), plus one test. The fix changes the exception type for a malformed reply. The cleanups keep every exception class and every message a caller could see.

Fix: non-UTF-8 replies. _read_line decoded the response line itself, outside every handler in _request_once. A reply with invalid UTF-8 therefore escaped as a raw UnicodeDecodeError instead of LeanRuntimeProtocolError, and the UnicodeDecodeError clause around json.loads could not run. _read_line now returns bytes, and the caller decodes them inside that clause. test_non_utf8_response_is_a_protocol_error fails on main with the UnicodeDecodeError and passes here.

Cleanups

• Request phases. _request_once set a dispatched flag before sendall and branched on it in shared handlers. The connect call has its own handlers, so apart from settimeout, every OSError the shared handlers saw came after dispatch. A try around sendall and the read now maps those failures, and the outer handler covers settimeout. The no-retry rule after dispatch is unchanged, and test_connected_send_failure_is_never_retried still pins it: it fails if the post-dispatch handler raises LeanRuntimeUnavailable. The "timed out waiting for Lean runtime connection" message is gone because it could not occur: connect's own OSError handler turned socket.timeout into "cannot connect to Lean runtime: ...".
• Errno fallback. The connect handler's errno in {2, 61, 111} check is gone. ENOENT and ECONNREFUSED already raise FileNotFoundError and ConnectionRefusedError, which the clause above catches. On Linux, 61 is ENODATA, which connect does not return.
• Runtime sources. _build_id and _build_generation listed the same five files; they now share _RUNTIME_SOURCES. _build_id still hashes servers/__init__.py first, so its input order is unchanged.

No open PR edits lean_client.py. #168, #111 and #105 edit tests/test_shared_lean_runtime.py; each merges cleanly with this branch, and the merged file passes with #111 (which contains #168, 62 passed) and with #105 (39 passed).

Validation at exact head 301a1df6: ruff check autoform_cli servers tests is clean, and tests/test_shared_lean_runtime.py passes (38). In tests/test_plugin_runtime.py, 3 pass; the fourth, test_wheel_contains_only_the_minimal_runtime, fails the same way on main in this checkout, because uv build reads a parent directory's pyproject.toml. CI runs it.