feat: zero network fees in relay execute flow

[matthewwalsh0]

Explanation

When using the Relay execute flow (EIP-7702 + execute enabled), network fees are subsidized by the relayer for cross-chain transactions. This change zeroes out the displayed source network fees in the quote normalization step so users see the correct zero cost.

For same-chain transactions, network fees are only zeroed when the quote includes a subsidized fee from the relayer.

References

• Builds on top of feat: support gasless relay deposits via execute endpoint #8133

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

---

Note

Medium Risk
Changes Relay quote normalization to report zero sourceNetwork fees under specific execute/subsidy conditions, which could impact fee display and downstream totals if the gating logic is wrong. Covered by new unit tests but still touches quote/fee computation paths.

Overview
Adjusts Relay quote normalization so fees.sourceNetwork is zeroed out when the Relay execute flow is active on EIP-7702 chains and the transaction is cross-chain, or when a subsidized fee is present (including same-chain).

Updates calculateSourceNetworkCost to accept an isSubsidized flag and short-circuit fee calculation with zero amounts (and empty gasLimits) under these conditions, and adds focused unit tests plus a changelog entry documenting the behavior change.

^{Written by Cursor Bugbot for commit 10fca5b. This will update automatically on new commits. Configure here.}