Fixes #6902 and #6903.

Two problems with stepWithRollback today:

• A step's undo is only registered after the step succeeds, so a step that has a side effect and then fails leaves nothing to roll it back (sdk(workflow): register a step's undo before running it, not after #6902).
• The undo reuses the step's own config, so you can't give rollbacks their own retry strategy (sdk(workflow): Give the undo its own config instead of reusing the step's #6903).

To avoid a breaking change, this adds stepWithRollbackV2 that fixes both. we now register the undo before the step runs, passes the result to the undo as TOutput | undefined. this way undo reconciles against real state when the step failed, and takes a separate undoConfig.

The old stepWithRollback is kept and marked @deprecated.

The workflow test file was also broken before this. it stubbed an aws.fetch export that doesn't exist, so the whole file threw on import and none of the tests ran (these SDK tests aren't in CI, so I guess it went unnoticed). The first commit fixes that by mocking awsFetch via mock.module instead, so the suite actually loads before the V2 work builds on it.