Resolve absolute dotenv paths without discovering a search directory

[FanWu-ai]

find_dotenv() documents that an absolute filename is returned as-is, but first tries to discover a search directory. If the working directory has been removed, find_dotenv('/absolute/path/.env', usecwd=True) raises FileNotFoundError even when that file exists. An absolute filename can also unnecessarily depend on caller-frame discovery.

Check fully absolute paths directly after expanding ~. Keep the existing file/FIFO check and missing-file return/exception behavior. Relative-path searching, including Windows rooted paths without a drive, stays on the original path.

Validation on Linux, Python 3.12.14:

• Original source: 10 new regression cases fail, 2 Windows-path compatibility cases pass.
• Final full suite: 300 passed, no skips (baseline: 288 passed).
• Tox lint: Ruff lint/format and all five mypy targets, Python 3.10–3.14, passed.
• Exact check-manifest, coverage reporting, both pinned pre-commit hooks, strict MkDocs build and git diff --check passed.
• Regressions include a real deleted working directory, expanded home paths, missing files, directories and FIFOs. Windows rooted-path behavior is exercised with standard-library Windows path semantics; this is not a native Windows run.

Prepared and tested with OpenAI Codex assistance, including an independent automated review. No human review or hosted CI pass is claimed.