fix: precheck session open funding

[brendanjryan]

Summary

• Validated session-open deposits before sponsored broadcast.
• Validated opening voucher amounts before sponsored broadcast.
• Added a shared assertion for session-open economic checks.

Motivation

Sponsored session-open transactions should only be broadcast after the deposit and voucher satisfy the requested payment amount.

Key design considerations

• The check runs inside the pre-broadcast hook before fee-payer gas can be spent.
• Existing descriptor and voucher signature validation remain unchanged.

[pkg-pr-new]

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npm i https://pkg.pr.new/mppx@583

commit: c92384a