save: structured data to .txt / unknown extensions should default-serialize or give an actionable error

[AMDphreak]

Describe the solution you'd like

When save gets structured pipeline data (tables/records) and the destination extension does not map to a to <format> serializer (e.g. .txt, no extension, or an unknown suffix), Nushell should not hard-fail with only Can't convert to string.

Prefer one of:

1. Default serialize structured input to a stable text format (json or nuon) and write it, ideally with a short stderr hint: note: no serializer for .txt; wrote JSON. Extension still overrides when known (.csv → to csv, etc.).
2. Or, if defaulting is too magical: keep the error but make it actionable — mention | to json | save … and | table | ansi strip | save … in the message.

Either way: people should not be forced to rename a file to *.json just to dump structured data. Filenames are hints; content is the point.

Describe alternatives you've considered

• Status quo + docs (ls | to json | save out.txt, ls | table | ansi strip | save out.txt) — works, still a paper cut.
• Always treat .txt as table render — matches the screen, but embeds ANSI unless stripped; bad for a data dump.
• Remove all extension-based auto-to (explicit only) — consistent, but regresses the nice save foo.json path.

Additional context

Related / prior:

• save file: can't convert record<...> to string #7233 — same cant_convert on save to .txt; closed as "use to json / table"
• Consider disabling save with a .html extension automatically invoking to html #7078 — pushback on too much auto-format from extensions (.html)

This is the other side of that coin: known extensions are helpful; unknown ones become a brick wall.

Interactive ls looks like text because the REPL pipes through table. Internal values are not JSON; .json triggers convert-then-write via to json. The asymmetry (output.json works, output.txt fails) is what feels overly strict.

ls | save -f output.txt   # fails: can't convert record<…> to string
ls | save -f output.json  # works

[fdncred]

I could support a better error message. Agreed that ls | save foo.txt should probably be calling to text similar to how ls | save foo.json calls to json. We'd probably accept PRs for those items.

You can also always pipe to table then to text to preserve ansi escapes. We don't want to lose that ls | table | save foo.txt.

You could also change your own display_output to do interesting things like always output json and not table.

No thanks to the default.