perf: Replace expensive calls in Table Plugin

[abimaelmartell]

Identified and optimized performance bottlenecks in GitHubFlavored table processing.

Bottlenecks Identified

1. isHeadingRow() — Line 155: parent.Children().First().IsSelection(s)

• Time: 9.19s (36.86% of total)
• Issue: Created new Selection objects and compared them for every row
• Fix: Compare node pointers directly, skipping text nodes

2. getCellContent() — Line 179: Linear search through parent's children

• Time: 90ms per call (called for every cell)
• Issue: O(n) search to find cell index
• Fix: Direct node pointer comparison to check if first child

3. isHeadingRow() — Line 149: Iterating through all children

• Issue: No early exit when non-TH found
• Fix: Break early when a non-TH cell is found

Existing tests are passing, i will add a test with a similar test case that was taking the 8s

[Copilot]

Pull request overview

This PR optimizes performance in the Table plugin by replacing expensive goquery operations with direct node pointer comparisons and early-break logic. The changes focus on reducing computational overhead when processing HTML table structures during markdown conversion.

Key Changes:

• Added early-break optimization when checking if table row cells are all TH elements
• Replaced parent.Children().First().IsSelection(s) with direct node pointer comparison in isHeadingRow
• Replaced linear search through children with direct node pointer comparison in getCellContent

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