feat(typescript): add optional builderCode to x402 client and server

[ethanoroshiba]

Summary

• Add optional builderCode on CdpX402Client and createX402Server to auto-attach the x402 builder-code extension (client s, server a)
• Omit the field → extension left unset; invalid codes rejected at init/create
• Docs, config schema, and unit coverage for inject / register / override paths

Test plan

• Unit tests for client + server builder-code wiring pass
• Client with builderCode attaches extension; without leaves it unset
• Server with builderCode injects on routes; without leaves unset; route extensions override works

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[ethanoroshiba]

Note on facilitator support: the CDP-hosted facilitator supports the builder-code extension, so app (a) and service (s) codes advertised/attached by createX402Server and CdpX402Client are encoded into the ERC-8021 Schema 2 calldata suffix at settlement. No self-hosted facilitator changes are needed to get attribution on the default path.

[phdargen]

Think we should change this and always attribute the client regardless of the server, dont see a good reason to drop

[phdargen]

Patched here: x402-foundation/x402#2994