feat(strategies): non-throwing parameter(T, default) accessor

[ocots]

Summary

• Adds Strategies.parameter(strategy_type, default), the non-throwing counterpart to parameter(strategy_type) — the same relationship as get(dict, key, default) to dict[key].
• default is only substituted when the underlying call throws Exceptions.NotImplemented; a strategy that legitimately implements parameter and returns nothing still returns nothing, not default, and any other exception still propagates unchanged.
• This lets any caller walking an AbstractStrategy it did not author (and cannot guarantee implements the optional parameter contract) query it safely, instead of hand-rolling try/catch e; e isa NotImplemented || rethrow(); nothing end — which is exactly what OptimalControl does today in helpers/print.jl.

Fixes #518

On the issue's two proposed shapes

The issue offered parameter(T, default) or parameter(T; strict::Bool=true), "either works". They're not actually equivalent in Julia: keyword arguments are resolved only after dispatch on positional types, against whichever method that dispatch selects. Every currently-implemented strategy overrides parameter with its own plain 1-arg method (no keyword declared), so the keyword form would throw MethodError: no keyword argument strict for every strategy that already implements the contract correctly, and only work for the ones that fall through to the abstract default (the ones that don't need the safety net in the first place). The 2-arg method has no such issue — it wraps a call to parameter(T) internally and works uniformly regardless of which concrete method that inner call resolves to. Went with the 2-arg form for this reason.

Test plan

• New testset in test/suite/strategies/test_abstract_strategy.jl:
  • falls back to default when parameter isn't implemented at all
  • a strategy returning a legitimate nothing is not conflated with the "unimplemented" case
  • a real parameter value passes through unchanged
  • a non-NotImplemented exception still propagates (not swallowed)
  • the 1-arg form's throwing behavior is unchanged
• suite/strategies/test_abstract_strategy.jl (90 tests) green
• Full suite green: 4962/4962

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