Memo preflight: validate memoless asset eligibility

Huginn Airequested to merge
huginn/2776-issue-k9nh2 into develop

Resolve #2776 (closed)

Summary

  • Add early validation to /thorchain/memo/check/{asset}/{amount} preflight endpoint to reject memoless-ineligible assets
  • Preflight now fails with explicit errors for:
    • Non-gas assets (e.g. ERC20 tokens) that require the chain router and cannot be deposited directly to Asgard vaults
    • When memoless transactions are globally halted (HaltMemoless mimir)
    • When memoless transactions are disabled (MemolessTxnTTL <= 0)
  • Preflight continues to pass for eligible gas assets (BTC, ETH, DOGE, etc.)
  • New test suite QuerierReferenceMemoSuite with 6 tests covering all validation paths

Test plan

  • Gas asset (BTC) passes preflight
  • Multiple gas assets (BTC, ETH, DOGE) pass preflight
  • ERC20 token (ETH.USDC) fails with "not eligible" error
  • Synth asset (BTC/BTC) fails with "not eligible" error
  • HaltMemoless mimir causes "currently halted" error
  • TTL=0 causes "currently disabled" error
  • Existing handler reference memo tests pass
  • make lint passes

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