fix: account for button height in read-more truncation threshold

[RsbhThakur]

Closes #13052

fix

Implements a robust line height buffer-based approach for the ol-read-more component's truncation logic. This replaces the fragile, absolute height-based truncation checks, ensuring the "Read More" toggle button only displays when collapsing the content saves a meaningful amount of vertical space.

Technical

• Modified _checkIfTruncationNeeded() in openlibrary/components/lit/OLReadMore.js to calculate overflow thresholds using line-height multiples rather than absolute pixel bounds.
• Added static BUFFER_LINES = 5 to represent the line buffer. The component only collapses if the hidden height would exceed the visible collapsed height by more than 5 lines (BUFFER_LINES). If collapsing the content would hide 5 lines or less, it is displayed fully with the toggle button hidden, ensuring the collapse always "earns its place".
• Implemented _getLineHeight(el) to dynamically resolve the computed line-height of the assigned slotted elements, falling back gracefully to fontSize * 1.5 if the CSS line-height evaluates to normal.
• Updated the Custom Elements Manifest (openlibrary/components/lit/custom-elements.json) to reflect the new component properties.
• Successfully verified code style and ran all pre-commit linter checks.

Testing

1. Edit the content of the <ol-read-more max-height="80px"> component in openlibrary/templates/design.html locally (or view any short/medium book description on /books/OL24620876M).
2. Verify that:
   • For text where the hidden portion is 5 lines or less, the "Read More" button is completely hidden and the content is fully shown.
   • For long text where collapsing it would hide more than 5 lines, the button displays normally and collapses the content to its configured max-height.

[lokesh]

Verified locally. LGTM!