fix: restore OAuth token expiry across process restarts - #3248

Problem

OAuthClientProvider._initialize (and the client-credentials providers) reloads current_tokens from storage but never restores token_expiry_time. The persisted OAuthToken only carries the relative expires_in, so on a fresh process is_token_valid() returns True for an already-expired access token — a stale Bearer is sent and a 401 round-trip is wasted before re-authentication (mcp2cli issues #50, #57).

Fix

Persist the absolute expiry and restore it on init:

• Add expires_at: float | None to OAuthToken (absolute unix timestamp), with a doc comment explaining the mcp2cli reproduction that motivated it.
• Set it when tokens are stored (_handle_token_response, _handle_refresh_response).
• Add OAuthContext.restore_token_expiry() and call it from all three _initialize methods (base, ClientCredentialsOAuthProvider, PrivateKeyJwtOAuthProvider).

Backwards compatible: expires_at defaults to None; existing stored tokens simply re-auth once, then persist the absolute expiry going forward.

Test

test_init_restores_expired_token_expiry — fails on main (expired token reported valid), passes with the fix. 210 auth tests pass, ruff + pyright clean.