docs: add runnable payment examples

[nkavian]

Summary

• Add runnable MPP and x402 Buyer/Seller examples using HTTPX and FastAPI.
• Document Sandbox account setup, commands, expected results, approval and cancellation, and existing upstream limitations.
• Exercise the actual payment middleware, free and paid responses, rejection, receipt handling, startup errors, and resource cleanup.
• Include examples in typing and coverage gates, the source distribution, and installed-wheel consumer checks.

Verification

make sync verify passed on Python 3.11, 3.12, 3.13, and 3.14, including formatting, strict typing, tests, distribution build, metadata validation, and isolated consumers.

No SDK runtime behavior changed. Tests use scripted platform responses; live Sandbox settlement was not performed.