feat(transaction-pay-controller): expose required transaction count

[dan437]

Explanation

Expose the expected number of TransactionController child transactions on Relay-backed payment quotes. The value uses the same Relay step parsing as submission, handles 7702 batches as one transaction, and stays undefined for unsupported paths.

This lets clients manage multi-signature hardware flows without parsing provider-specific quote data.

References

• Consumer: feat: dismiss Ledger prompt after deposit signing (CONF-1715) metamask-mobile#34872

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them