fix(templates): ignore escaped braces when collecting fixture keys

[rudycelekli]

What and why

The placeholder regex treated escaped literal braces as required fixture keys and omitted conversion/format-spec placeholders. Use Python Formatter parsing so fixture-key validation follows the rendering contract, including nested format specifications.

Reproduction and verification

• 3 failing contract cases and 2 passing controls before the fix. Six focused regression cases pass after adding nested-format coverage.
• Focused local pytest result:

......                                                                   [100%]
6 passed in 0.01s

• Ruff 0.16.9, Bandit (-r ifixai -ll), layout validation and all 11 bundled example-fixture validations passed locally.
• Tested on Python 3.13; upstream CI has not yet run for this branch.

Scope

Malformed format strings continue to fail as malformed templates rather than being silently accepted.

Prepared with AI assistance; the regression reproduction and changed source were reviewed before publication.