test: verify Python and Node payment interoperability

[nkavian]

Summary

• Exercise Python Buyers against Node Sellers and the reverse over real local HTTP.
• Cover MPP charges and Buyer subscriptions, x402 balance/exact payments, pending approvals, rejection, settlement failure, and handler failure.
• Assert request counts, credential separation, lifecycle order, and receipts, with corrupted-receipt negative controls.
• Pin the Node source revision and upload reports from Python 3.11–3.14 jobs.

Verification

• Full repository checks passed locally on Python 3.11 and 3.14.
• All 49 payment cases and four negative controls passed on both versions.
• No production SDK behavior or dependencies changed.

The platform is synthetic; these checks do not make live payments. Python MPP Seller subscriptions remain excluded under the documented upstream limitation.