fix(csv-stringify): preserve objects returned by cast functions

[agammann]

When a custom cast function returns the same { value, quoted } object for multiple fields, the first field deletes its value property. Subsequent fields become empty. For example, two fields cast to the same quoted "shared" value produce only one populated field.

Copy the cast result before extracting and deleting the local value option. Add regression coverage for reusing a result and for returning a frozen result.

Validation on Node.js 24.19.0:

• The reused-result regression fails before the fix.
• On Linux, the csv-stringify TypeScript check and complete Mocha test glob pass: 211 passing, 1 pending.
• ESLint passes for both changed files.