macOS Save throws errSecDuplicateItem (-25299) when another writer creates the same item between find and add #183

What's wrong

MacOsCredentialStore.Save (CredentialCache/Storage/MacOsCredentialStore.cs:81-129) is find-then-modify-or-add: it calls SecKeychainFindGenericPasswordWithRef, and on errSecItemNotFound calls SecKeychainAddGenericPassword, throwing CredentialStoreException on any non-zero add status (126-129).

That's a check-then-act race. If another writer creates the same service/account item after the find but before the add, the add returns errSecDuplicateItem (-25299) and Save throws. The per-persona lock in CredentialCache.cs (~:144, :201) only serializes within one CredentialCache instance, so it doesn't cover two processes, two cache instances, or direct ICredentialStore users.

The Windows (CredWrite) and Linux (secret_password_store_sync) stores are both native upserts and can't fail this way, so on macOS alone ICredentialStore.Save isn't reliably the "store or replace" it's documented as.

Failure scenario

Two processes (or two MacOsCredentialStore instances) save the same persona for the first time concurrently. Both finds return not-found; the second add fails and the caller gets CredentialStoreException: SecKeychainAddGenericPassword failed (-25299). (Found by reading the code; not run on macOS.)

Suggested fix / acceptance criteria

• Add errSecDuplicateItem = -25299 to NativeMethods.
• When the add returns errSecDuplicateItem, retry the find-by-ref + SecKeychainItemModifyAttributesAndData branch once, then throw only if that also fails.
• Concurrent Save calls on separate instances never throw, and the stored value afterwards equals one of the writes.
• Cover the retry branch with a test (e.g. the IL-scan style used by NativeStoreScrubbingWiringTests, or a seam around the add status).

Note: open PR #182 (for #166) modifies the same Save method, so whoever picks this up should build on top of it.