Track successful next_call completion separately from invocation so execution
middleware that catches and translates a downstream provider/tool failure does
not accidentally convert that failure into a successful None result.
Also avoid wrapping BaseException from downstream execution, and document the
execution middleware error semantics.
Tests cover:
- pre-next_call middleware failures fail open to the remaining chain
- post-next_call middleware failures preserve the downstream result
- translated downstream failures propagate instead of returning None
- downstream BaseException is not wrapped
Signed-off-by: Bryan Bednarski <bbednarski@nvidia.com>
