feat(cli): stream non-TTY exec input before EOF

[shiju-nv]

Summary

Add sandbox exec --stream-stdin so a client can exchange requests and responses with one remote process before closing stdin. The mode runs without a TTY and keeps stdout and stderr separate.

Related Issue

Closes #3802

Changes

• Reuse the bidirectional exec RPC while preserving default finite-input behavior.
• Retain the 4 MiB total stdin limit; cancel on overflow and warn that earlier input may already have been processed.
• Drain output through the final gRPC status, report trailing transport errors, and never automatically retry an interrupted command. Update exec documentation and CLI guidance.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)