Track remaining MultiSwapper quoteOut and SwapSource regressions

Summary

Track the two remaining MultiSwapper regressions introduced by #158 that are intentionally not fixed in #167.

Remaining Regressions

1. swap(nil, ...) / swapBack(nil, ...) can become inconsistent with quoteOut(...).

   Example:

   • caller provides 10 TokenA
   • selected inner capped route reports quoteOut(10) = { inAmount: 4, outAmount: 4 }
   • MultiSwapper._swap(...) still falls back to quoteOut(from.balance) and forwards the full 10 TokenA vault to the selected inner swapper
   • strict inner swappers can reject the call because execution forwarded more input than quote.inAmount

2. SwapSource.withdrawAvailable(maxAmount) can return more than maxAmount.

   Example:

   • caller asks for withdrawAvailable(maxAmount: 10)
   • selected route returns quoteIn(10) = { inAmount: 10, outAmount: 11 }
   • SwapSource forwards that quote into swap(...)
   • caller receives more than the requested maximum amount

Notes

• #167 intentionally fixes only the partial-route quoteIn(...) tie-break regression.
• This follow-up issue keeps the larger quoteOut/execution-contract discussion and the SwapSource max-amount behavior together in one place.

Start by comparing the MultiSwapper regressions described here with issues #158 and #167, then trace the swap(nil, ...), swapBack(nil, ...), quoteOut(...), and SwapSource.withdrawAvailable(maxAmount) paths. Reproduce both examples and define completion as execution matching the quote and withdrawAvailable never returning more than maxAmount.