fix: always send required freeOnly on price constraint

[filippofinke]

Summary

Price-range searches (.price({ min, max })) failed with HTTP 422 GRAPHQL_VALIDATION_FAILED at path prices[0].freeOnly. Root cause: the tutti GraphQL input type ListingPriceConstraint declares freeOnly as a required non-null Boolean, but SearchBuilder.price() only spread the caller's {min,max} and omitted it.

HTTP 422 Unprocessable Entity: {"errors":[{"message":"Internal Server Error","path":["variable","constraints","prices",0,"freeOnly"],...,"code":"GRAPHQL_VALIDATION_FAILED"}],"data":null}

.freeOnly() worked because it set the field explicitly; .price() did not.

Root cause (verified, not inferred)

Decompiled app ListingPriceConstraint input adapter:

• freeOnly:Z (primitive boolean) + checkNotNull → required, always written
• key: String! → required
• min / max → Optional<Double> → optional

Real input shape:

input ListingPriceConstraint { freeOnly: Boolean!, key: String!, min: Float, max: Float }

Changes

• src/search/builder.ts — default freeOnly: false in .price() (before the spread, so an explicit value still wins)
• src/core/types.ts — make freeOnly required on PriceConstraint so the omission can't recur; comment cites the adapter
• README.md — price shape is now verified against the decompiled adapter (removed from the "inferred" caveat)

Verification

Fake-transport check of the variables .fetch() sends:

PASS price({max}):     {"key":"price","freeOnly":false,"max":5000}
PASS price({min,max}): {"key":"price","freeOnly":false,"min":100,"max":5000}
PASS freeOnly():       {"key":"price","freeOnly":true}