test: SequencerFeeRow coverage + sequencer getBlockNumber fragility fix

[mike-diamond]

What

Closes the test-coverage gaps from the Buildathon work (item 3, post #22). Two logical changes, two commits:

1. fix(arbitrum-adapter) - previewSequencerFee no longer discards a valid fee when getBlockNumber rejects. previewBlock is informational metadata, so the block lookup is now best-effort in its own try/catch and degrades to undefined; an RPC hiccup on the block height can no longer null out a good gasEstimateComponents estimate. Written TDD (red, then green).
2. test(example) - first jsdom + testing-library vitest setup in the repo, plus a SequencerFeeRow spec covering its three render states.

Tests

• @txkit/arbitrum-adapter: 24 passing (was 23, +1 fragility test) + typecheck clean
• @txkit/example-arbitrum-london: 3 passing (new suite, fully mocked - no RPC, no contracts)
• Full pnpm test (turbo): 15/15 tasks green - the example suite now runs in CI

Coverage notes

Covered:

• previewSequencerFee block-number resilience (the fix above)
• SequencerFeeRow: estimating hint while in flight / renders nothing when the preview is null / L1 + L2 + total rows when it resolves with data

Consciously skipped (YAGNI for the hackathon):

• PolicyStatusBadge, ChatMessage - trivial presentational render, near-zero ROI, covered by the visual demo
• EnvelopePreview / CopyableValue render tests - optional, lower priority, deferred
• SequencerFeeRow chainId hardcode (the in-browser feeClient is pinned to Arbitrum Sepolia) - left as-is: the demo is Sepolia-only and the component JSDoc says so. The new test pins current behavior, so deriving the client from the chain prop later becomes a deliberate, test-visible change.

Notes

• The example gains a test script, so it joins pnpm test under turbo in CI. The spec mocks previewSequencerFee, so it is deterministic and needs no network.
• Uses @vitejs/plugin-react-swc for the JSX transform (lighter peer footprint than the babel plugin under vite 8). It prints a cosmetic "switch to @vitejs/plugin-react" perf hint - harmless for 3 tests.
• Pre-existing array-bracket-spacing nits on untouched lines in the adapter spec are not touched here (out of scope; the adapter's CI lint is tsc, not eslint).

Not merging - leaving open for review.

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[socket-security]

Warning

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Action	Severity	Alert  (click "▶" to expand/collapse)
Warn		Obfuscated code: npm entities is 91.0% likely obfuscated Confidence: 0.91 Location: Package overview From: pnpm-lock.yaml → npm/entities@4.5.0 ℹ Read more on: This package | This alert | What is obfuscated code? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Packages should not obfuscate their code. Consider not using packages with obfuscated code. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/entities@4.5.0. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.
Warn		Obfuscated code: npm entities is 91.0% likely obfuscated Confidence: 0.91 Location: Package overview From: pnpm-lock.yaml → npm/@astrojs/mdx@5.0.4 → npm/entities@6.0.1 ℹ Read more on: This package | This alert | What is obfuscated code? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Packages should not obfuscate their code. Consider not using packages with obfuscated code. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/entities@6.0.1. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.

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