fix(ton-trading-bot): surface per-DEX errors in get_arbitrage_opportunities

[konard]

Problem

ton_trading_get_arbitrage_opportunities consistently fails with the generic error "Could not fetch DEX quotes" (fixes #102).

The root cause was two-fold:

1. The function called sdk.ton.dex.quote() (aggregated) but that method's failure was silently caught and turned into a null — the actual error was only logged as a warning, invisible to the caller.
2. dexFees listed tonco and swapcoffee as expected DEX keys, but the SDK only exposes quoteSTONfi and quoteDeDust — so those entries could never be populated.

Fix

• Switch from the aggregated sdk.ton.dex.quote() to individual sdk.ton.dex.quoteSTONfi() / sdk.ton.dex.quoteDeDust() calls run in parallel.
• Each failure is logged with its specific error and the affected asset pair (e.g. StonFi quote failed for TON→USDT: Router contract not found).
• When all DEXes fail, the error response now includes per-DEX detail: Could not fetch DEX quotes — stonfi: …; dedust: ….
• When only one DEX fails, the function continues with the partial result (one quote is still enough to surface partial data — arbitrage just isn't possible with fewer than 2 quotes, which is already handled downstream).
• Remove tonco / swapcoffee from dexFees — the SDK does not expose quote methods for those DEXes.

How to reproduce

// Before fix — always fails silently:
sdk.ton.dex.quote = async () => { throw new Error("Router contract not found") }
// result: { success: false, error: "Could not fetch DEX quotes" }  ← no detail

// After fix — actual error is surfaced:
// result: { success: false, error: "Could not fetch DEX quotes — stonfi: Router contract not found; dedust: …" }

Tests added

Test	What it verifies
error message includes the actual DEX error, not just a generic message	The response includes per-DEX failure details, not just the old generic string
succeeds with partial quotes when only one DEX fails	Function returns success: true with an empty opportunities list rather than failing completely when one DEX is unavailable

All 106 tests pass locally (node --test plugins/ton-trading-bot/tests/index.test.js).

🤖 Generated with Claude Code

[konard]

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[konard]

✅ Ready to merge

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