feat(cdp): support DOM.scrollIntoViewIfNeeded

[lisa0314]

Summary

• implement DOM.scrollIntoViewIfNeeded for nodeId, backendNodeId, and objectId
• invoke the existing element scroll shim so subsequent input hit testing uses the resolved target
• reject missing, unknown, and non-scrollable nodes instead of returning a false success

Validation

• cargo nextest run -p obscura-cdp (98/98)
• cargo build --release
• obstacle course (33/33, 12.0 ms median latency)
• git diff --check

Addresses the DOM.scrollIntoViewIfNeeded portion of #531.