refactor(historical): use Subscription<HistoricalBarUpdate> (closes #431)

[wboayue]

Summary

• Replace HistoricalDataStreamingSubscription with Subscription<HistoricalBarUpdate> by adding a StreamDecoder<HistoricalBarUpdate> impl that mirrors the MarketDepths / Bar / Trade pattern in src/market_data/realtime/.
• Factory functions in both historical/sync.rs and historical/async.rs collapse to encode + builder.send::<HistoricalBarUpdate>(). Custom struct, Drop, and inline message dispatch deleted in both files.
• Net -271 lines (132 insertions, 403 deletions).

Closes #431.

Why

The custom subscription duplicated infrastructure that the generic Subscription<T> already provides: AtomicBool cancelled guard, message_bus storage, cancel encoding, tokio::spawn in async Drop, error storage, timezone-fallback resolution, and inline message-type dispatch. Each lived in two places (sync + async).

After the refactor each piece has one job:

• HistoricalBarUpdate::decode — message-type dispatch.
• per-message decode_historical_data{,_update,_end} — payload parsing (already split, reused).
• HistoricalBarUpdate::cancel_message — cancel encoding.
• Subscription<T> — lifecycle (cancel, drop dedup, error storage).
• factory — request encoding + send.

HistoricalBarUpdate joins the same plug-in family as Bar, Trade, BidAsk, MarketDepths — all consumed by the same generic Subscription<T>.

Breaking changes (intentional, v3)

• Async: subscription.next().await now returns Option<Result<HistoricalBarUpdate, Error>>. The separate error() accessor is gone — errors arrive via next().
• Sync: unchanged in this PR (still Option<HistoricalBarUpdate> + error()). Aligning sync with async is tracked in Align sync Subscription<T>::next() with async: return Option<Result<T, Error>> #487.
• HistoricalDataStreamingSubscription is no longer exported. Callers use Subscription<HistoricalBarUpdate> directly. Per the v3 philosophy in CLAUDE.md, no type alias.

Test plan

• cargo fmt
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --features sync -- -D warnings
• cargo clippy --all-features
• cargo test (default/async): 880 lib + 73 doc + integration — all green
• cargo test --no-default-features --features sync: 884 + 124 — all green
• cargo test --all-features: 1064 + 148 — all green
• cargo build --example async_historical_data — clean
• Manual against IB Gateway paper trading: verify Historical → End → ongoing Updates with keep_up_to_date=true, and clean Ctrl-C cancel

Behavioral assertions verified by tests

• Drop with no prior cancel → exactly one CancelHistoricalData proto sent (factory-driven test).
• Explicit cancel().await then drop → exactly one cancel sent (dedup via Subscription's AtomicBool).
• Error response → next() yields Some(Err(_)) (async) / None + error() populated (sync).