DorkFiMCP: detect borrow cap and fail early in borrow_txn

[temptemp3]

Summary

DorkFiMCP's borrow_txn (and related borrow flows) should detect when a borrow would exceed the market borrow cap and fail early with a clear error, instead of building/signing and potentially failing on-chain or giving a confusing result.

Problem

When a user calls borrow_txn with an amount that would push total borrowed (e.g. existing borrows + accrued interest + new borrow) over the market's borrow cap:

• The MCP may build transactions that will fail on-chain or behave unexpectedly
• Users get a poor experience (e.g. signing then failure, or unclear reason for failure)
• No upfront validation against the borrow cap in the MCP layer

Expected behavior

• Before building borrow_txn, DorkFiMCP should:
  1. Resolve the market's borrow cap (if any) and current total borrowed / user borrow (including accrued interest where applicable)
  2. Compute whether current borrowed + borrow amount would exceed the cap
  3. If over cap: return a clear error (e.g. "Borrow would exceed market cap: current X, cap Y, requested Z") and do not return transaction bytes
• If there is no cap or cap is not applicable, proceed as today

Suggested implementation

• In the code path that handles borrow_txn (and any wrapper that uses it):
  • Fetch market info (e.g. via get_market or equivalent) including borrow cap and current borrowed amounts (principal + interest)
  • Optionally fetch user's current borrow in that market
  • Validate (user_current_borrow + borrow_amount) and/or (total_borrowed + borrow_amount) against cap
  • If over cap, return a structured error and no txn payload
• Add or extend MCP/unit tests for: at-cap, over-cap, and no-cap scenarios

Context

Improves UX and reliability for DorkFi MCP callers; aligns with borrow cap behavior in the app (see e.g. DorkFi/dorkfi-app#248). Complements early deposit-cap check in UluOS#7.