feat(integration): add bounded x402 buyer

[epistemedeus]

Proposed Changes

Add an optional buyer-side x402 adapter that reuses the official x402 HTTP
transport while keeping purchase authority in the calling application.

• add uagents-adapter[x402] with the official x402[evm,httpx] dependency;
• require an application-owned request, origin and method allowlists, a maximum
  per-payment amount, and a mandatory output validator;
• reject caller policies or selectors that could run after the SDK spend cap;
• require selected terms to match the seller's original challenge and permit at
  most one payment creation per logical purchase;
• bound every identity-encoded response below the x402 buffering layer, reject
  compressible encodings, disable redirects, and require a valid idempotency key
  for explicitly enabled state-changing methods;
• preserve delivery, selected terms, settlement response, and ambiguous
  post-signature attempt evidence without presenting native seller evidence as
  independent chain verification; and
• add hostile boundary tests plus a locked adapter-specific CI job so the x402
  suite cannot be silently skipped by the root environment.

Linked Issues

Related to #771.

Types of changes

• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Documentation update.
• Something else (tests and CI).

Checklist

• I have read the CONTRIBUTING guide
• Checks and tests pass locally

If applicable

• I have added tests that prove my fix is effective or that my feature works
• I have added/updated the documentation

Further comments

This is intentionally a narrow buyer adapter. It does not create wallets,
expose a generic message-driven payment proxy, add seller middleware, perform
service discovery, or claim independent settlement verification.

Local verification on Python 3.12:

• 32 passed in the adapter x402 hostile suite;
• Ruff lint and formatting checks pass;
• locked x402 dependency sync resolves successfully;
• source distribution and wheel build successfully; and
• the upstream Python suite reports 107 passed, 1 skipped.

No live payment was made while testing this contribution.