fix(sdk-coin-ton): return MPCSweepTxs from recover() for unsigned sweeps

[vibhavgo]

Summary

`recover()` was returning a bare `MPCTx` for unsigned sweeps instead of the `MPCSweepTxs` format that OVC's TSS form requires. The TSS form reads `txRequest.walletCoin` to determine the MPC algorithm — the bare return caused silent upload failure at the start of the 5-pass OVC ceremony.

Other coins already follow this convention — see `sdk-coin-polyx` `polyx.ts:263` which returns `{ txRequests: [txRequest] }`. This PR aligns TON with that established pattern.

Linear: WCI-680

Changes

• `modules/sdk-coin-ton/src/ton.ts`: wrap `isUnsignedSweep` return value in `MPCSweepTxs` shape
• `modules/sdk-coin-ton/test/unit/ton.ts`: add real (non-stub) test calling `recover()` with mocked network that asserts the full `MPCSweepTxs` structure — mirrors the test pattern in `sdk-coin-polyx`

Test Plan

• `BITGOJS_TEST_PASSWORD=test npx mocha --grep "should generate unsigned sweep in MPCSweepTxs"` — 1 passing
• Full `yarn unit-test` in `modules/sdk-coin-ton` passes
• End-to-end: fresh TTON unsigned sweep from WRW uploads successfully to OVC 5-pass ceremony and broadcasts successfully on testnet

[linear-code]

WCI-680

[vibhavgo]

Reference: Other coins returning MPCSweepTxs from recover()

TON's recover() was the only MPCv2 EdDSA coin that returned a bare MPCTx instead of wrapping it in the MPCSweepTxs envelope. The following coins follow the correct pattern this PR aligns TON with:

• DOT (sdk-coin-dot): https://github.com/BitGo/BitGoJS/blob/ea9a5cbd1d/modules/sdk-coin-dot/src/dot.ts#L477-L478
• POLYX (sdk-coin-polyx): https://github.com/BitGo/BitGoJS/blob/ea9a5cbd1d/modules/sdk-coin-polyx/src/polyx.ts#L265-L266
• NEAR (sdk-coin-near): https://github.com/BitGo/BitGoJS/blob/ea9a5cbd1d/modules/sdk-coin-near/src/near.ts
• SOL (sdk-coin-sol): https://github.com/BitGo/BitGoJS/blob/ea9a5cbd1d/modules/sdk-coin-sol/src/sol.ts

OVC's sign-tss-first-trip-form requires txRequest.walletCoin on the parsed result — the { txRequests: [{ walletCoin, transactions: [...] }] } shape is what parseJsonTransactions routes to the TSS path. Without this wrapping, upload silently fails in the OVC UI.

[github-actions]

⚠️ Unit tests are failing on Node 26.x (Current release line, non-blocking). This is not an LTS version yet, so it does not block merge, but it signals an incompatibility to fix before Node 26.x becomes LTS.

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