Quote API: enforce memoless eligibility and reject non-gas source assets

Closing this Issue because it's invalid - based on the incorrect assumption that "memoless ONLY supports gas assets, not ERC-20s".

Context (Incident Follow-up)

This issue is created in reaction to the recent memoless / direct-Asgard deposit incident discussed in THORChain Discord (InstaSwap user case, Feb 25–27, 2026): https://discord.com/channels/838986635756044328/1476157453736476703

Related follow-up issues from the same incident:

• #2776 - Memo preflight: validate memoless asset eligibility
• #2777 - EVM direct Asgard ERC20 deposits: add safe auto-refund

Problem

/thorchain/quote/swap can return a valid-looking quote for swaps that are not valid in memoless mode (ERC20 source asset like ETH.USDT). This allows integrators to misroute users into deposits that cannot execute memoless.

Proposal

• Add an explicit quote mode flag, e.g. mode=memoless.
• If mode=memoless and source asset is not memoless-eligible (currently gas asset only), return a clear error.
• Add explicit response metadata for integrators:
  • memoless_supported: bool
  • memoless_reason: string (optional but useful for UX/messages)
• Keep backward compatibility when mode is omitted.

Acceptance Criteria

• ETH.USDT -> SOL.SOL with mode=memoless is rejected with clear error.
• ETH.ETH -> SOL.SOL with mode=memoless succeeds.
• API docs updated with memoless eligibility semantics.