fix(csv-stringify): preserve caller cast configuration

[sricharanreddycheruku]

A frozen cast configuration fails when normalization writes default callbacks into it. Copy the configuration before filling defaults, so a caller can reuse the same frozen object.

The TypeScript regression uses should.js, runs the synchronous API twice and verifies that the original configuration is unchanged.

Local validation on Ubuntu, Node 24.19.0:

• Complete csv-stringify tests and TypeScript check: 233 passing, one existing pending test.
• The new frozen-configuration test fails against unchanged production code and passes after the fix.
• Package build passes. Lint has no errors and one existing pending-test warning.
• Formatting, normal commit hooks and git diff --check pass.

The preceding workspace run hit an unchanged csv-parse test expecting "an boolean" while the source now says "a boolean". That failure was reproduced on the unchanged parser. This contribution leaves that package unchanged. Other Node versions and browser runtime checks were not run locally.