[joho/godotenv] Align quoted-value escaping with dotenv behavior (issue #241)

Job description

Fix godotenv issue 241: https://github.com/joho/godotenv/issues/241 Make double quoted value parsing conform to the documented dotenv behavior discussed in the issue, with focused regression tests for the affected escaping cases. Keep the parser change narrow and avoid unrelated compatibility changes. Deliverables: upstream PR linked to the issue, changed files summary, and test results. The PR body or a linked issue comment must mention this OpenJobs listing URL.