Fix curly bracket spacing around curly f-string expressions

[MichaReiser]

Summary

Fixes #15459

F-string expression need to leave one space between the element's { or } and any curly braces inside the expression itself or it results in invalid syntax:

f"{ {1, 2, 3} }"  # valid
f"{{ 1, 2, 3}}"  # invalid

The existing implementation handles this correctly if the expression itself has curly braces, but it doesn't add the necessary space if the expression starts or ends with an expression with curly parentheses:

-print(f"{ {1, 2, 3} - {2} }")
-print(f"{ {1: 2}.keys() }")
+print(f"{{1, 2, 3} - {2}}")
+print(f"{{1: 2}.keys()}")

This PR changes the formatter to check if the left-most sub expression (if any) starts with curly parentheses instead of just checking the expression itself.

Test Plan

Added snapshot tests.

[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

[dhruvmanila]

Thank you!