docs: align safe-integer justification with tools.mdx

The x-mcp-header integer bound is justified as "the safe range for JavaScript" here and as the IEEE754 double-precision range in server/tools.mdx. Same constraint, same numeric range, two different justifications. The IEEE754 phrasing was agreed in the review of #2772; the revision landed in tools.mdx but not here.

No normative change: the bound is untouched.