test(dotenv): cover reported quote newline round trips

[fibibot]

Closes bartlomieju/orchid-inbox#69

Summary

• add explicit regression coverage for the four reported dotenv stringify/parse round-trip failures
• keep the broader tricky-character round-trip test and clean up its issue reference wording

Testing

• deno fmt --check
• deno lint
• deno test --allow-read --allow-env dotenv/stringify_test.ts dotenv/parse_test.ts
• deno test -A --parallel --trace-leaks --coverage --doc --clean dotenv
• deno task test, failed in unrelated suites in this VM: ANSI color expectations under fmt/internal/cli despite NO_COLOR=1, fs walk regex counts, and a CBOR test allocating more than 4 GiB

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (95a1e2e) to head (92c30eb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7145   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         634      634           
  Lines       51843    51843           
  Branches     9346     9346           
=======================================
  Hits        49050    49050           
  Misses       2218     2218           
  Partials      575      575           

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