[`ruff`] Ignore `fstring-missing-syntax` (`RUF027`) for `fastAPI` paths (#12939)
## Summary
As suggested by @MichaReiser in
https://github.com/astral-sh/ruff/pull/12886#pullrequestreview-2237679793,
this adds an exemption to `RUF027` for `fastAPI` paths, which require
template strings rather than eagerly evaluated f-strings.
## Test Plan
I added a fixture that causes Ruff to emit a false-positive error on
`main` but no longer does with this PR.