fix: resolve CWD before lru_cache key in find_project_root

[anisia19]

Description

When srcs is empty (e.g. black --code), find_project_root fell back to os.getcwd() inside the @lru_cache-decorated function. The working directory was never part of the cache key — only (srcs, stdin_filename) was — so two calls from different directories with srcs=() would share the same cache entry and return the wrong project root and pyproject.toml.

This was observable in the test suite: test_code_option_config and test_code_option_parent_config both call black --code from different directories using change_directory. Whichever ran second would hit the stale cache entry from the first and load the wrong config.

Fix: split the function into two parts:

• find_project_root (public, no cache) resolves the CWD fallback upfront and normalises all srcs to absolute paths before handing off.
• _find_project_root_cached (private, @lru_cache) only ever receives fully-resolved paths, so its cache key is stable regardless of where the process is running from.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? — N/A, no formatting changes.
• Add an entry in CHANGES.md if necessary? — Yes, added under Configuration.
• Add / update tests if necessary? — The existing test_code_option_config and test_code_option_parent_config tests cover this exactly; they were failing before the fix.
• Add new / update outdated documentation? — N/A, no user-visible API or formatting behaviour changed.