Fix/unilateral exit preconfirmed tree sig

[d4rp4t]

Stacks on top of #117

Problem

Starting a unilateral exit for a VTXO whose tree-tx isn't MuSig2-co-signed yet
(PSBT_IN_TAP_KEY_SIG absent) failed the exit session immediately:

Tree tx input 0 is missing the MuSig2 taproot key-path signature
(PSBT_IN_TAP_KEY_SIG); arkd should have populated it during the batch round.

This happens for any VTXO that is still preconfirmed — arkd only co-signs the
tree during a batch round, so the signature is absent until the batch fires. The
SDK made it worse: VtxoChainAutoFetchService fetches the branch on the first
VtxosChanged event (while the VTXO is preconfirmed, so the tree-tx PSBT is
unsigned) and VirtualTxService.FetchAndStoreBranchAsync short-circuits on
HasBranchAsync, so the stale unsigned branch is never refreshed.

Fix (SDK)

Mirror go-sdk's ErrWaitingForConfirmation retry loop — wait for the tree to be
co-signed instead of failing:

• VirtualTxService.FetchAndStoreBranchAsync gains a forceRefresh flag that
  bypasses the HasBranchAsync short-circuit, so a stale (unsigned) branch can
  be re-pulled from the indexer.
• UnilateralExitService.ParseVirtualTx now throws a typed, transient
  TreeTxNotSignedException for the missing-signature case.
• UnilateralExitService.ProgressBroadcastingAsync catches it, force-refreshes
  the branch, and leaves the session in Broadcasting to retry on the next
  ProgressExitsAsync — instead of moving it to Failed.

No behaviour change for already-committed VTXOs.

Tests

Moves the two exit tests blocked by this gap out of #117 (chore/e2e-coverage)
to this branch, and un-ignores the preconfirmed one:

Mirror go-sdk's ErrWaitingForConfirmation retry loop — wait for the tree to be
co-signed instead of failing:

• VirtualTxService.FetchAndStoreBranchAsync gains a forceRefresh flag that
  bypasses the HasBranchAsync short-circuit, so a stale (unsigned) branch can
  be re-pulled from the indexer.
• UnilateralExitService.ParseVirtualTx now throws a typed, transient
  TreeTxNotSignedException for the missing-signature case.
• UnilateralExitService.ProgressBroadcastingAsync catches it, force-refreshes
  the branch, and leaves the session in Broadcasting to retry on the next
  ProgressExitsAsync — instead of moving it to Failed.

No behaviour change for already-committed VTXOs.

Tests

Moves the two exit tests blocked by this gap out of #117 (chore/e2e-coverage)
to this branch, and un-ignores the preconfirmed one:

• ProgressExits_WorksForOffchainFundedVtxo
• ProgressExits_WorksForPreconfirmedVtxo (was [Ignore]d on the same blocker)

Verified against the local regtest: the exit no longer fails on the missing
signature — it correctly detects the unsigned tree, refreshes the branch, and
waits (Broadcasting, not Failed).

⚠️ Open item: an idle preconfirmed VTXO is not auto-committed by arkd — the
owner must settle it (join a batch) to get the tree co-signed. Both tests
therefore need a settlement step before/around the exit (reusing the
SettleAVtxoAsync machinery) to go fully green. Tracking that here before merge.