fix(clerk_auth): authenticate delete requests and preserve failed deletions

[funkydevil]

Summary

• authenticate both DELETE /client and DELETE /me before clearing cached credentials
• keep local sign-out behavior when the network request fails
• preserve the authenticated session when account deletion fails
• surface failed account deletion instead of returning Client.empty
• clear the persisted client only after successful account deletion

Problem

_delete cleared TokenCache before building the request. As a result, the
Authorization header and _clerk_session_id query parameter were missing
from DELETE /me. Clerk could return a response without deleting the user,
while the SDK replaced its local client with an empty client and reported
success.

Moving the cache clear into an unconditional finally, as proposed in #446,
correctly authenticates the request and preserves offline sign-out behavior.
However, account deletion has different failure semantics: if DELETE /me
fails, clearing the cache turns that failure into a local sign-out and the
caller can still believe that the account was deleted.

This change lets _delete distinguish the two operations:

• signOut() always clears local credentials after attempting the request.
• deleteUser() retains credentials on failure, throws an ExternalError,
  and clears them only after a successful response.

Tests

• verifies that sign-out sends the client token and then clears it
• verifies that user deletion sends both the client token and session ID
• verifies that successful deletion clears credentials
• verifies that failed deletion throws and preserves the signed-in user
• preserves the existing offline sign-out tests

Local verification:

• dart analyze
• dart test — 674 passed, 1 skipped because it requires a configured Web3 wallet

Related to #446.