## Why
An MCP tool call can run after its selected environment becomes ready, while the
turn context still contains the earlier starting-state snapshot. File argument
rewriting must use the environment available to the current step so it can
resolve and upload local files in that case.
## What changed
- Pass `StepContext` through MCP file argument rewriting.
- Resolve the primary environment from the step snapshot while retaining
turn-scoped configuration and metadata.
- Add coverage for an environment that transitions from starting to ready
during a turn.
GitOrigin-RevId: 935bea18ee425aabeacf4505eaff90565f9751e9