Python bindings for compound transactions

[wjones127]

Parent PRD

Milestone: Action-based Transactions (UserOperation) — see milestone description for full PRD.
Discussion: #5960

What to build

Expose compound transaction construction in the Python SDK. Thin wrapper over the Rust API.

Acceptance criteria

• Python API for constructing and committing compound transactions
• Tests mirroring the Rust integration tests
• Documentation with examples

Blocked by

• Blocked by Compound transaction Rust API #6457 (compound transaction Rust API)

User stories addressed

• User story 7: construct compound transactions programmatically