deleteCredentials throws on missing entry, causing re-auth error loops

[nbardy]

Problem

KeychainTokenStorage.deleteCredentials() and FileTokenStorage.deleteCredentials() throw an error when the credential doesn't exist. This causes cascading "Failed to clear OAuth credentials" errors when re-auth tries to clear credentials that are already gone.

Common scenarios:

• Double logout (/auth logout twice)
• Auth-switch without prior login (clearing credentials that were never stored)
• Token refresh race (keychain entry deleted externally between read and delete)

Expected behavior

Deleting non-existent credentials should be a no-op (idempotent), not an error. This matches standard idempotent-delete semantics and the behavior users expect from a "clear" operation.

Affected code

• KeychainTokenStorage.deleteCredentials() — throws No credentials found for ${serverName}
• FileTokenStorage.deleteCredentials() — throws No credentials found for ${serverName}

Environment

• Affects all keychain users (macOS/Linux) for Google OAuth logout path
• Affects GEMINI_FORCE_ENCRYPTED_FILE_STORAGE=true users for MCP token path

[nbardy]

Fixed in #21762

[mvanhorn]

I've submitted a fix for this in PR #21949. Makes deleteCredentials silently succeed when credentials don't exist, preventing the cascading error loops.