Make the price extension a faithful decorator

[loevgaard]

Three ways the decoration could silently drop or corrupt behaviour. None of them misbehave against Sylius 1.12 today — they are all "the moment X changes, this breaks quietly", which is the worst way for a price calculation to break.

1. Rebuilt filters lost their options. new TwigFilter($name, $closure) discarded whatever options the original carried. The three Sylius filters declare none, so nothing is lost right now, but escaping (is_safe, pre_escape, preserves_safety) and argument passing (needs_environment, needs_context, is_variadic) would have silently changed the moment one of them did. copyOptions() now carries them across.

is_safe is the interesting one: it cannot be read back directly, and getSafe() needs the argument Node, which only exists at compile time. Forwarding an is_safe_callback that defers to the original getSafe() preserves both is_safe and is_safe_callback correctly and at the right moment.

Two options are deliberately not copied, both commented in the code: node_class (a filter compiled through a custom node would not route through our callable at all, so copying it would produce a filter that silently ignores the VAT context) and the deprecation version/alternative detail (only readable via methods Twig has itself deprecated — the notice survives, the detail does not).

2. __call was decorative. It looked like the remaining extension methods were forwarded to the decorated extension. It never fires for any of them: AbstractExtension declares getFunctions(), getTests(), getTokenParsers() and getNodeVisitors(), so the parent's empty values won and anything the base extension grew would have vanished. They are delegated explicitly now and __call is gone.

getOperators() is the one exception and it has a comment saying why — Twig 3.21 annotates it with classes it has since removed, which any override inherits, and psalm rejects the result. The decorated extension registers no operators, so inheriting AbstractExtension's [[], []] is identical in behaviour.

3. The price context was assumed to be argument 2. That stops being true the moment a filter asks for the environment or the template context, since Twig prepends those — the wrapper would then have written vat_context_aware into the variant argument. The index is now derived from the filter itself.

Also adds PriceExtensionTest, which the decorator previously had none of. It drives the filters through a real BasePriceExtension/PriceHelper and asserts the calculator actually receives vat_context_aware, that sylius_has_discount is passed through un-rebuilt, and that the remaining methods delegate.

Addresses the third architecture item in #5.

https://claude.ai/code/session_01P9NzuVPQGvaVR97HZFq98a