Typo'd paths silently yield empty configs in ConfigReader and extract_exports (split from #16, items 5 and 6)

[thorwhalen]

Split out of #16 (items 5 and 6, same root cause: detect-and-fall-through where detect-and-raise would catch user error).

Reproduction (verified 2026-09-26, config2py 0.1.54)

>>> from config2py import extract_exports, ConfigReader
>>> extract_exports("path/to/.env")          # file does not exist
{}
>>> r = ConfigReader("no/such/file.ini")     # file does not exist
>>> dict(r), r.source_kind
({'DEFAULT': <Section: DEFAULT>}, 'filepath')

Note on item 5: the typo'd-path case goes through the isinstance(source, str) / no-newline branch of ConfigReader.__init__ (self.read(source), which silently skips missing files), not the final else branch quoted in #16. Both deserve a fix.

Why this was not changed unattended

Raising where we currently return empty is a behaviour change for existing callers (for instance code that probes optional config files with ConfigReader(path) and expects an empty result). The docstrings now state the current behaviour (cloud-sweep PR).

Proposed plan

1. ConfigReader: when a no-newline str/Path source does not exist, raise FileNotFoundError, unless a new keyword-only missing_ok=False flag is set to True. In the final else branch raise TypeError(f"Unsupported source: {type(source).__name__}").
2. extract_exports: if the argument has no newline, does not start with export , and is not an existing file, raise FileNotFoundError. A single export A="1" line keeps working.
3. Tests for each case, then the dependents gate (py2store re-exports config2py.s_configparser).