fix: Support for 9 fractional second digits (ns)

[YousafZahid1]

Closes #341

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

Closes #1218

Took 4 attempts to find a passing fix — worth a careful review.
Low confidence (30%) — recommend a thorough review.

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Draft — promote to ready when satisfied.