fix: forward x402 v2 extensions in payment payload for Bazaar discovery

[oceans404]

Summary

Fixes #196

I was building Dermi (a skincare/makeup ingredient checker API) for the OWS hackathon and wanted to get it listed on the x402 Bazaar discovery layer. I had the v2 Bazaar extension configured correctly on the server side, and payments were going through the CDP facilitator successfully — but Dermi never appeared in the Bazaar.

After debugging, I found that OWS parses accepts and resource from the 402 PAYMENT-REQUIRED header but drops the extensions field. Since the facilitator reads extensions.bazaar from the payment payload during settle to catalog services, the discovery metadata was never reaching the facilitator.

Once I added extensions support to OWS, Dermi was indexed on both the CDP Bazaar and PayAI Bazaar immediately.

Changes

crates/ows-pay/src/types.rs

• Add extensions: Option<serde_json::Value> to X402Response (so extensions are parsed from the 402 header)
• Add extensions: Option<serde_json::Value> to PaymentPayloadV2 (so extensions are included in the payment sent back)

crates/ows-pay/src/x402.rs

• parse_requirements now returns extensions as a 4th value
• handle_x402 passes extensions through to build_signed_payment
• build_signed_payment passes extensions to build_evm_exact
• build_evm_exact includes extensions in the PaymentPayloadV2

Test plan

• All 62 existing tests pass (cargo test -p ows-pay)
• Tested against live Dermi endpoint on Base mainnet — payment succeeds and service appears in CDP Bazaar
• Tested against PayAI facilitator — service appears with full inputSchema/outputSchema metadata
• Backwards compatible — extensions is Option with serde(default), so 402 responses without extensions still work

[vercel]

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