[FastAPI] Avoid introducing invalid syntax in fix for fast-api-non-annotated-dependency (FAST002)

[arkuhn]

Summary

• Context: this rule swaps out default function argument values for type annotations
• Problem: When this swap occurs after a default argument value is already present, the function signature is no longer valid python code
• Fix: Track default arguments and do not recommend a fix if we hit this condition, making this fix only sometimes possible.

Fixes #12982

Addtl:

• Breakout helper functions

Test Plan

# 3 new test cases demonstrating default value scenarios
cargo run -p ruff -- check crates/ruff_linter/resources/test/fixtures/fastapi/FAST002.py  --no-cache --preview --select FAST002

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[AlexWaygood]

Thanks! This seems like a reasonable fix to me

[codspeed-hq]

CodSpeed Performance Report

Merging #13133 will improve performances by 7.61%

_{Comparing arkuhn:akuhn/FAST002_autofix (8e2aaf3) with main (2e75cfb)}

Summary

⚡ 1 improvements
✅ 31 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	arkuhn:akuhn/FAST002_autofix	Change
⚡	linter/all-rules[numpy/globals.py]	784.7 µs	729.2 µs	+7.61%