GH-38217: [R] Map format = "text to format = "csv" in write_dataset()

[thisisnic]

Rationale for this change

write_dataset() errors on format="text" even though open_dataset() allows it.

What changes are included in this PR?

Map "text" to "csv" so write_dataset() mirrors open_dataset()

Are these changes tested?

Yes

Are there any user-facing changes?

No

• GitHub Issue: [R] Map format = "text to format = "csv" in write_dataset() #38217

[github-actions]

⚠️ GitHub issue #38217 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

This PR aligns the R write_dataset() format handling with open_dataset() by treating format = "text" as an alias of CSV, so users can write delimited text datasets with the same expectations as when opening them.

Changes:

• Map format = "text" to "csv" inside write_dataset() so it uses CSV defaults (notably comma delimiter) and generates .csv filenames.
• Narrow the “delimiter required” validation to only apply to format = "txt", avoiding the previous error for format = "text".
• Update tests to cover the new format = "text" behavior and remove the old expectation that it errors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
r/R/dataset-write.R	Adds text -> csv mapping and updates delimiter validation to only enforce a delimiter for txt.
r/tests/testthat/test-dataset-write.R	Replaces the previous error expectation for format="text" with a round-trip test asserting .csv output and correct data.