docs: 402 body now mirrors the x402 challenge (x402Version, accepts)

[VickyXAI]

Summary

BlockRun's gateway now spreads x402Version/accepts at the top level of every 402 JSON body, in addition to the signed PAYMENT-REQUIRED/X-Payment-Required/WWW-Authenticate headers — see BlockRunAI/blockrun#TBD for the code change. Pre-v2-era x402 clients (early x402-fetch/x402-axios, third-party wrappers) only ever parsed the body and, finding no top-level accepts, silently gave up instead of auto-paying.

This PR brings every doc's 402 example in line with the new body shape:

• docs/api-reference/errors.md — canonical 402 shape + new explanatory paragraph
• docs/x402/payment-flow.md — the "402 Response" step now shows the mirrored fields, with a note on what the headers carry that the body mirror omits (resource, extensions)
• docs/x402/how-it-works.md — one-line update to the "requirements are base64-encoded..." paragraph
• All 12 endpoint reference pages with a 402 example (chat-completions, defillama, exa-search, image-generation, modal-sandbox, polymarket-funding, realface, responses, search, text-to-speech, video-generation, virtual-portrait) + music-generation

While touching docs/api-reference/responses.md I also picked up an upstream fix already on main (fee-inclusive price.amount wording) via the merge — no separate action needed, just noting it so the diff isn't a surprise.

Not in this PR

.gstack/* local browse-session artifacts have unrelated pre-existing local diffs on this machine and are left out.

Test plan

• Spot-check the rendered docs site for the touched pages
• Confirm the blockrun PR (code + tests) merges around the same time so the two stay in sync

[VickyXAI]

Closing as already shipped: this branch was the base of #68, so its commits went to main inside that squash (6b4c84f) — docs/x402/how-it-works.md and docs/x402/payment-flow.md are live. Nothing here is lost; reopen if you see anything missing.