fix: preserve Tempo fee-payer-signed fields

[struong]

Summary

Preserves filled Tempo transaction fields after eth_fillTransaction returns a fee payer signature.

Motivation

Caller-provided gas, fee, or nonce preferences could overwrite fields selected and signed by a fee payer, producing a final transaction whose payload no longer matched the fee payer signature.

Changes

• Skipped caller preference and fee multiplier processing once a non-null fee payer signature is present.
• Preserved unsigned eth_fillTransaction behavior.
• Added focused fill-boundary coverage and a Tempo transport regression for the final signed transaction path.
• Added a patch changeset.

Testing

• ./node_modules/.bin/biome check --write src/actions/public/fillTransaction.ts src/actions/public/fillTransaction.test.ts src/tempo/Transport.test.ts
• ./node_modules/.bin/tsc -b
• ./node_modules/.bin/vitest run --config test/vitest.config.ts src/actions/public/fillTransaction.test.ts -t "fee-payer-signed|unsigned relay"
• Tempo transport regression was attempted locally, but shared node setup returned HTTP 400 for eth_getBlockByNumber before tests executed.

[changeset-bot]

🦋 Changeset detected

Latest commit: bc0dd62

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
viem	Patch

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Project	Deployment	Actions	Updated (UTC)
viem	Ready	Preview	Aug 16, 2026 10:54pm

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