Clear stale orders on bankruptcy

[OllyNewport]

Addresses three remaining items from #1318 (following #1389 and the earlier docs PR for #5/#17/#20).

#6 — Bankruptcy leaves stale orders

_Broker.next() already force-closes every open Trade when equity drops to <= 0
(which also cleans up their SL/TP orders), but any still-pending, unfilled order
(e.g. a limit order that never triggered) was left dangling in Strategy.orders /
_Broker.orders, even though the simulation has stopped and it can never execute.
self.orders.clear() is now called alongside the rest of the bankruptcy cleanup.

Covered by a new regression test that drives an over-leveraged position into a 99%
crash and asserts stats._strategy.orders == () afterward.

#18 — Type hints polish (Trade, Order, Backtest.optimize)

Filled in missing return-type (and a couple of parameter-type) annotations across
Order's and Trade's properties/methods, and tightened Backtest.optimize's
method parameter to Literal['grid', 'sambo'].

• Trade.sl / Trade.tp setters were typed float but the docstring always allowed
  None to cancel the order — retyped to Optional[float].
• Annotating the sl / tp getters as -> Optional[float] surfaced a real mypy
  ambiguity in self.__sl_order and self.__sl_order.stop (mypy can't prove Order
  is always truthy). Rewrote both as an equivalent ternary — no behavior change,
  just type-checker-friendly.

No logic changes elsewhere.

#19 — Order.__repr__ verbosity

Order.__repr__ always printed contingent=... even when is_contingent was
False — and rendered it as contingent=0 rather than contingent=False, because
round(False, 5) == 0. Falsy contingent is now omitted like any other unset
field, while a genuinely contingent order still shows it (e.g.
<Order size=-1.0, stop=90.0, contingent=1>).

Covered by a new test asserting 'contingent' is absent from a plain order's repr
and present on a contingent SL order's repr.

Testing

• python -m backtesting.test — 83 passed, 1 skipped (unrelated)
• flake8 backtesting — clean
• mypy backtesting — same 9 pre-existing errors as master (all in unrelated
  files: _stats.py, _plotting.py, lib.py); zero new errors, two errors in
  Trade actually fixed as a side effect of the sl/tp rewrite above

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