Fix unstable f-string formatting for expressions containing a trailing comma

[MichaReiser]

Summary

Fixes #15536

The RemoveSoftlinesBuffer removes soft line breaks and if_group_breaks elements but it
doesn't remove expand_parent elements. This can lead to instable formatting
when f-strings are wrapped in a group because it makes that group expand. For example,
print(f"{ {}, 1, }") first gets formatted to

print(
    f"{ {}, 1 }"
)

and only then converges at print(f"{ {}, 1 }").

I first tried to also remove expand_parent elemnents in the RemoveSoftLinesBuffer
but that results in many failing tests around assignment formatting because
we rely on Document::will_break to determine if we should try the flat
layout and that only returns true if the f-string formatting contains the expand_parent along the trailing comma.

I reviewed all expand_parent usages and realized that the trailing comma is the only
usage in our expression formatting. That's why I decided to adjust the trailing comma
formatting instead to omit the trailing comma if we know that we're in a flat f-string.

This is also how Dhruv implemented it originally before I removed the check in #14489 because I thought it
it no longer necessary.

Test Plan

Added regression test

[github-actions]

ruff-ecosystem results

Formatter (stable)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

warning: Detected debug build without --no-cache.
error: Failed to read examples/Assistants_API_overview_python.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: expected `,` or `]` at line 197 column 8

Formatter (preview)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

ruff format --preview

warning: Detected debug build without --no-cache.
error: Failed to read examples/Assistants_API_overview_python.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: expected `,` or `]` at line 197 column 8