pass every preflight field to the rules engine

[bpapillon]

Prepares the C# SDK for the v0.10.0 rules engine's event_quantities preflight, which prices a call (requests plus token counts) the way the API burns it. Mirrors schematic-go #250 as far as this SDK's hand-written code allows.

Local evaluation already hands the generated PreflightRequestBody to the engine as its options, but HasPreflight checked a fixed list of knobs, so a field the type gains on regen would have been silently left off. It now sends whatever the body serializes, minus null and empty-object fields, so event_quantities reaches the engine as soon as Fern adds it.

The generated rules-engine models keep unknown fields as extension data, so quantity_rates on conditions and entitlements already reaches the engine and comes back on the result. The new tests pin both, and run the API's shared quantity_cost.json fixture through the WASM engine with event_quantities carried in the body's extension data.

There's no public way to set event_quantities yet: this SDK's only preflight surface is the generated PreflightRequestBody (reached through Check), and that type gets the field once the API adds it to the preflight body and Fern regenerates. Exposing it on CheckOptions is a follow-up after that.