fix: MemMapFs.OpenFile checks parent dir exists before creating file

[veeceey]

Fixes #270

MemMapFs.OpenFile with O_CREATE was silently creating parent directories that don't exist. This meant WriteFile would happily write to non-existent paths in MemMapFs while OsFs would correctly return an error -- inconsistent behavior.

The fix adds a parent directory existence check in OpenFile before calling Create. If the parent doesn't exist, it returns os.ErrNotExist, matching os.OpenFile semantics.

Updated existing tests that relied on the implicit directory creation to explicitly create directories first (which is what you'd need to do on a real filesystem anyway). Added a dedicated test covering the bug.

[CLAassistant]

All committers have signed the CLA.

[veeceey]

Test Results

Full test suite passes:

$ go test -count=1 ./...
ok  	github.com/spf13/afero	4.528s
ok  	github.com/spf13/afero/mem	0.195s
ok  	github.com/spf13/afero/tarfs	0.648s
ok  	github.com/spf13/afero/zipfs	0.506s

Verified the fix with the new test TestOpenFileNonExistentDirectory:

$ go test -count=1 -v -run TestOpenFileNonExistentDirectory ./...
=== RUN   TestOpenFileNonExistentDirectory
--- PASS: TestOpenFileNonExistentDirectory (0.00s)

Also confirmed manually that WriteFile to a non-existent directory now returns os.ErrNotExist, matching OsFs behavior.

[veeceey]

gentle ping when you have a moment