Avoid curly brace escape in f-string format spec

[dhruvmanila]

Summary

This PR fixes a bug in the lexer for f-string format spec where it would consider the {{ (double curly braces) as an escape pattern.

This is not the case as evident by the PEP as well but I missed the part:

[..]

• If in “format specifier mode” (see step 3), an opening brace ({) or a closing brace (}).
• If not in “format specifier mode” (see step 3), an opening brace ({) or a closing brace (}) that is not immediately followed by another opening/closing brace.

Test Plan

Add a test case to verify the fix and update the snapshot.

fixes: #7778

[dhruvmanila]

Current dependencies on/for this PR:

• main
  • PR Avoid curly brace escape in f-string format spec #7780 👈

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[codspeed-hq]

CodSpeed Performance Report

Merging #7780 will improve performances by 2.94%

_{Comparing dhruv/no-double-curly-escape-in-format-spec (6d6c8e1) with main (c040fac)}

Summary

⚡ 1 improvements
✅ 24 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dhruv/no-double-curly-escape-in-format-spec	Change
⚡	linter/default-rules[pydantic/types.py]	38.9 ms	37.8 ms	+2.94%

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.