fix: Estimate relay transactions separately and combine with original transaction gas at quote time

[dan437]

Explanation

References

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

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Note

Medium Risk
Touches gas estimation and fee calculation logic for Relay post-quote flows, including EIP-7702 batching; incorrect combining could misprice fees or submit with mismatched gas limits, but changes are scoped and well-covered by unit tests.

Overview
Post-quote Relay quoting now estimates gas for relay transactions only, then separately combines the original transaction gas into fees.sourceNetwork totals and returned metamask.gasLimits so quote-time costs match what relay-submit will actually batch.

This introduces combinePostQuoteGas to prepend original gas for non-7702 flows, add it into the single combined gas limit for EIP-7702 batches, prefer nestedTransactions[].gas over txParams.gas, and skip combining when the original tx has no to or no gas. Tests were updated/expanded to cover these post-quote behaviors and the fallback-gas path, and the package changelog notes the fix.

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