fix: prevent log loading from scrolling ancestor elements in task Logs tab

[Pyasma]

Closes: #64757

Problem

When switching to the Logs tab on a task instance with many log lines, scrollIntoView() was called on a sentinel <div> at the bottom of the log content. This API walks up the DOM tree and scrolls every scrollable ancestor until the target is visible — causing the details panel (and page) to scroll irreversibly, not just the log box itself.

Fix

Replace scrollIntoView() with a direct scrollTop = scrollHeight assignment on the log box element (logBoxRef). This only scrolls the log box itself, leaving all ancestor elements untouched.

Also updated useOffsetTop to accept RefObject<HTMLElement | null> to align with the stricter ref types in newer React/TypeScript versions.

Changes

• airflow/www/static/js/dag/details/taskInstance/Logs/LogBlock.tsx — replaced scrollIntoView with direct scrollTop scroll; removed unused sentinel <div>
• airflow/www/static/js/utils/useOffsetTop.ts — updated parameter type to RefObject<HTMLElement | null>

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (claude-sonnet-4-6) following the guidelines

[Pyasma]

Closing in favour of #64889 which has a cleaner diff (no unrelated file).