fix: propagate refresh token persistence errors

[he-yufeng]

Summary

Fixes #2034.

When refresh-token flow succeeds but provider.saveTokens() fails, the current auth() path catches that persistence error in the same try block as refreshAuthorization(). That makes the SDK fall through to a fresh authorization redirect even though the authorization server may already have rotated the refresh token.

This separates the refresh request from token persistence. Refresh failures keep the existing fallback behavior, while persistence failures now propagate to the caller so filesystem or storage problems are visible and the client does not silently lose the newly issued tokens.

To verify

• pnpm --filter @modelcontextprotocol/client exec vitest run test/client/auth.test.ts -t "propagates token persistence errors"
• pnpm --filter @modelcontextprotocol/client exec vitest run test/client/auth.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint
• git diff --check
• pre-push hook: full workspace typecheck, build, and lint

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