Use Intl.NumberFormat for token formatting in SessionsInsights

[aharvard]

Addresses feedback from #6449

Replaces custom token formatting logic with Intl.NumberFormat using compact notation as suggested by @DOsinga.

Before:

const formatTokens = (tokens: number | undefined): string => {
  if (!tokens) return '0';
  if (tokens >= 1_000_000_000) return `${(tokens / 1_000_000_000).toFixed(2)}B`;
  if (tokens >= 1_000_000) return `${(tokens / 1_000_000).toFixed(2)}M`;
  if (tokens >= 1_000) return `${(tokens / 1_000).toFixed(1)}K`;
  return tokens.toString();
};

After:

const formatTokens = (tokens: number | undefined): string => {
  return new Intl.NumberFormat('en', { notation: 'compact', maximumFractionDigits: 2 }).format(
    tokens || 0
  );
};

This is cleaner, more maintainable, and leverages the browser's native formatting capabilities.

[Copilot]

Pull request overview

This PR refactors token display formatting in the SessionsInsights component by replacing custom conditional logic with the browser's native Intl.NumberFormat API using compact notation.

Changes:

• Simplified the formatTokens function from 13 lines of conditional logic to a single-line call using Intl.NumberFormat with compact notation