[codex] Handle CredWrite logon session failures

[SirAndrosBot]

What changed

• Added KeyStorageException, a presentable exception type for user-facing key-storage failures.
• Mapped CredWrite ERROR_NO_SUCH_LOGON_SESSION / 0x520 to a clear warning message when Windows Credential Manager cannot save the database key.
• Added elevated-process guidance to that warning when KeePass is running as Administrator.
• Kept all other CredWrite failures on the existing reportable error path.
• Updated the legacy KeePassWinHello.csproj so the new exception is included in PLGX/legacy builds.

Why

Issue #82 reports repeated generic bug-report dialogs from CredWrite error 0x520 while persistent storage is enabled and KeePass is running elevated.

Windows error 0x520 is ERROR_NO_SUCH_LOGON_SESSION: Windows Credential Manager cannot access the relevant logon session for the write. In this case the plugin should not tell users this is an unexpected internal bug; it should explain that the persistent key was not saved and point at the elevated-process condition when applicable.

Behavior notes

This PR deliberately does not disable Credential Manager storage, delete persistent Windows Hello keys, or silently fall back to in-memory storage. A first worker pass tried that, but review rejected it as too broad for a possibly transient storage write failure. The final patch is warning-only: it preserves existing persistent-storage state and keeps the failure visible.

Refs #82

Validation

• Worker subagent implemented the initial fix in an isolated branch.
• Reviewer subagent rejected the first version because it disabled persistent storage and could delete persistent auth material.
• Patch was trimmed to warning-only behavior.
• Reviewer subagent approved the trimmed version with no findings.
• git diff --check passed with only Git LF-to-CRLF working-copy warnings.
• MSBuild.exe src\KeePassWinHello.csproj /t:Rebuild /p:Configuration=Release /p:DefineConstants=MONO /p:FrameworkPathOverride=C:\Windows\Microsoft.NET\Framework\v4.0.30319 /p:ReferencePath=C:\proj\KeePassWinHello\lib passed with 0 warnings and 0 errors.

Manual test needed

• Elevated KeePass with persistent storage enabled, reproducing CredWrite 0x520, to confirm the warning appears instead of the generic report dialog.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud