fix: sponsored access key gas estimation

[parvahuja]

prepareTransactionRequest still runs Tempo's afterFillParameters hook when callers pass parameters: []. The hook currently turns missing gas into a 10,000 or 20,000 placeholder, so a later eth_fillTransaction treats that placeholder as an explicit gas limit instead of estimating gas.

Only apply the existing access-key/WebAuthn buffer when gas has already been filled. This preserves the original buffers and the relay-signed gas behavior.

Regression coverage verifies that:

• missing gas remains missing for a sponsored access-key micro-prepare
• an existing 33,000 gas estimate still receives the 10,000 access-key buffer

Live verification reproduced the failure as a 20,000 gas limit for a transaction requiring 43,828, then completed the same OpenClaw payment with HTTP 200 after applying this guard.

Validation:

• focused regression tests: 2 passed
• pnpm build:esm
• Biome check on changed source and tests

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[changeset-bot]

🦋 Changeset detected

Latest commit: 63481b2

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

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