market_data/historical/sync: lift coverage 60.92% → 72.09% (#565)

Add 28 tests covering version gates (head_timestamp, historical_schedule +
trading-class arm, ticks_{bid_ask,mid_point,trade}, cancel_historical_ticks,
histogram_data, historical_data_streaming_trading_class); error / EOS arms
(head_timestamp unexpected/EOS, historical_data HISTORICAL_DATA_END follow-on,
type-4 Error response, schedule unexpected/EOS, histogram empty); the
previously untested historical_schedules_ending_now and cancel_historical_ticks
surfaces; and TickSubscription internals (try_next/next_timeout drains,
unexpected-message skip, fill_buffer Err via RoutedItem::Error channel
injection, fill_buffer None on closed channel, bid_ask/midpoint try_iter +
timeout_iter monomorphizations).

Adds two sync test helpers (assert_version_check_fails /
assert_unexpected_response) mirroring the async-file pattern from PR #563.

Real-code coverage (without `--doctests` phantom-instrumentation): 94% lines,
100% functions, 92% regions. The `just cover` ceiling for this file is set by
9 `no_run` doc-examples that get instrumented but never execute — same shape
as contracts/sync/mod.rs and other sync siblings with full doc coverage.