refactor(challenge): build_accepted_methods + build_how_to_pay consume *RailSpec; async

[vvillait88]

Summary

`build_accepted_methods` and `build_how_to_pay` now accept `*RailSpec` instances directly instead of plain dicts. The helpers are async because `RecipientLike` supports async factories (used by Stripe-multichain merchants that mint fresh deposit addresses per PaymentIntent).

Single canonical type per rail: vendors declare one `TempoRailSpec` / `X402BaseRailSpec` / `SolanaMppRailSpec` / `StripeRailSpec` / `TempoSessionRailSpec` and pass the same instance to every helper.

rails = {
    "tempo": TempoRailSpec(recipient=tempo_addr),
    "x402_base": X402BaseRailSpec(recipient=base_addr),
    "solana_mpp": SolanaMppRailSpec(recipient=solana_addr),
    "stripe": StripeRailSpec(profile_id=os.environ["STRIPE_PROFILE_ID"]),
}
accepted = await build_accepted_methods(**rails)
how_to_pay = await build_how_to_pay(url=..., retry_body_json=..., total_usd=..., rails=rails)

`build_how_to_pay` does not resolve `RecipientLike` factories — the block surfaces CLI commands, none of which include the recipient string. `build_accepted_methods` does resolve, since each entry carries `pay_to`.

Migrates the existing test suite + `multi_rail_merchant` example inline. Other examples (api_provider, variable_cost_merchant, ...) migrate in follow-up PRs.

Test plan

• `uv run pytest tests/` — 1048 passed, 95.09% coverage
• `uv run ty check` — clean
• `uv run ruff check` — clean

🤖 Generated with Claude Code