`PipelineData::into_value` implicitly waits for `ExternalStream` to finish and irreversibly converts it

Related problem

For example, you can run this command:

~> run-external --redirect-combine cmd /c "echo test" | complete
╭───────────┬──────╮
│ stdout    │ test │
│           │      │
│ exit_code │ 0    │
╰───────────┴──────╯

But, if you were to have an intermediate pipeline step that does not attempt to change the pipeline data, like inspect or describe:

~> run-external --redirect-combine cmd /c "echo test" | describe | complete
Error:   × Complete only works with external streams
   ╭─[entry #2:1:1]
 1 │ run-external --redirect-combine cmd /c "echo test" | describe | complete
   ·                                                                 ────┬───
   ·                                                                     ╰── complete only works on external streams
   ╰────

Any attempt to convert the pipeline data into a value causes the consuming command to block until it is done, and irreversibly converts the data into a string.

This is caused by this logic:
https://github.com/nushell/nushell/blob/903afda6d946960682dc5234724a09d2bfed288b/crates/nu-protocol/src/pipeline_data.rs#L156-L209

Describe the solution you'd like

It's surprising that interpreting the pipeline data associated with an external command causes it to:

1. Be transformed
2. Block until the external command has finished executing

For my use case, I'm trying to run an external command, collect its output and echo its output to nushell's stdout, and wait for the command to finish to get its exit code.
I cannot do all three at the same time.

As for the solution, I'm not sure. Perhaps it would be worth considering a new value type for an external command, or to convert the external stream into a record and back into an external stream for use in a command like collect.

Describe alternatives you've considered

No response

Additional context and details

No response

Start by reading the PipelineData::into_value logic in crates/nu-protocol/src/pipeline_data.rs around lines 156-209, then reproduce the behavior with run-external, inspect or describe, and complete. Define and test behavior that preserves the external stream while allowing output collection and exit-code handling; the issue does not specify the final representation or implementation.